Wrong first impressions. (POSSIBLE SPOILERS!)

[OminousFlare]

Madoka - I think this is pretty much the same with everyone else. Cute premise, deceiving appearance, Episode 3, total mindblown. Episode 8 made me love the series with all my heart, even though I was already hooked by the varied aspect given to the Magical Girl genre. And of course, it convinced me that there's no light of happiness down that tunnel.

Fractale - Expected it to be Nausicaa or, at least, Moving Castle. Turned out to be contrived and cliche. :\

Freezing - Had hoped for the characters to face the more realistic angsty some of the reviews (of the manga) had claimed (before the anime was even broadcast). I knew the story was going to be shounen-level all along, but turns out that the characters were not much more to watch than those in hentai, either.

Hourou Musuko - Expected this to be deeper, more socially challenging. Turned out its premise was more probing than its content. Good stuff as a seinen anime, though.

K-On!! Season 2 - Mio. 'Nuff said.

Gintama - As much as I love this awesome stuff now, I never expected it to be this entertaining, considering that I'm not a big fan of medieval times, ninjas, and samurais.

Evangelion - Thought it'd be nerdy mecha in the tradition of Star Trek, but the human drama actually got me sold. The villains were annoying at first, but after "The End of Eva," I have not much to complain.

[Progbuusters]

Samurai7 started off really good, but then the original illustrators took a break and the tweeners took over the artwork. Looked really bad. Plus the story flailed and the heroes became unlikeable.

[Archon_Wing]

Kanon The first episode was god awful, and all those verbal tics killed me. It wasn't until a rewatching where I got introduced to the sword wielding, demon fighting, moe female Jesus goddess and the girl who wouldn't let such petty things such as death and freezing weather get between her and ice cream that I realized it was worth watching.

Yosuga no Sora I thought it was gonna have lots of explicit fanservice, but only a tease. I was half right.

Madoka I thought it would be Nanoha on LSD with an incomprehensible plot. Thankfully the plot was comprehensible.

Okami Kakushi- I thought the show would be good, because Deen didn't do it; and it looked good. I was sorely mistaken

Ore no Imouto I thought it would have been a story where a brother would realize his sister wasn't really a bitch and would be cute. I was also sorely mistaken; that role went to a black cat.

[~Yami~]

well, some people always say about School Days (I don't have any courage to watch it... lol)

but I will certainly agree about Madoka Magica.... absolutely unexpected.....
I can say the same about Clannad and Angel Beats....

I also have a gundam-like impression when I watch Code Geass but when I watched the ending I realized that I'm wrong.....

love this kind of anime so much.... lol

[Ithekro]

Nichijou - perhaps. Episode 00 gives off a slow slice of life feel. Not too much humor or absurdity. Episodes 01 and 02? Random absurdist comedy.

[stelok]

I thought Card Captor Sakura was a show for only little girls but it turned out to be able to entertain both adults and kids

[HasuMasu]

i don't often have wrong impressions, but i'll list the slippery ones that took me by surprise

Gundam Wing
- with all the praise i saw, i was not expecting it to be so bad, well played, well played...

The Melancholy of Haruhi Suzumiya
- same deal as Gundam Wing, i got all excited for well, nothing really

Grave of the Fireflies
- everybody i asked said it would be depressing, and so i was the victim of a massive trolling

well that's all i can gather right now, what with my horrible memory and all

they're mostly just shows that i expected to like, but didn't really

[Ichuki]

Magical Pokkan I thought it was gonna be a serious series judging by the opening but its not

[Haladflire65]

Originally Posted by OminousFlare

Gintama - As much as I love this awesome stuff now, I never expected it to be this entertaining, considering that I'm not a big fan of medieval times, ninjas, and samurais.

Seconding Gintama, but not for that reason... I didn't expect such epicness from the serious arcs after all the comedy episodes, not that I didn't like them or anything, but I think my mind was blown by the time I reached the Benizakura arc (then again at all the others - Yagyu, Shinsengumi Rebellion, Yoshiwara Burning...) Kind of sad, since so many people get into the show expecting samurai action then get turned off by the comedy, and not stick around till the serious arcs...

[Kotohono]

One that I was wrong about and much for the better...
Astarotte no Omocha! I got the expression from the premise it was going to be rather fan service heavy and probably plotless with typical tsundere kugumiya rie being violent towards the male. I could haven't been more mistaken the show not only has a plot with not unreasonable fan service and plenty of heartwarming moments, Kugumiya Rie's character, Lotte, almost never uses violence .

And then Let's first impressions the way that turns out badly...
Mayoi Neko Overrun! - the first three episodes made me think it would have a good story though out, I was mistaken . Since the random near meaningless episodes started soon after.

[Bonta Kun]

I would have probably agreed with Oreimo, if it wasn't for reading the manga first.
Oddly I never got the usual impressions I would have thought I would with it instead I did honestly think it was just about brother and sister improving their relationship.
I do think if I never read the manga and LN to start with I would have had the same first impressions as some others here.
How I guage series between manga and anime can vary.

Suzumiya Haruhi caught me out tho, I did think it would just be some slice of life, romcom nonsense but isn't quite the case

Asobi ni Iku Yo - Actually not sure about this one, I think I just never knew what to expect tbh, maybe had a few ideas but whatever they were I was surprised by how things turned out.

Ookiku Furikabutte - This series I didn't so much get a wrong impression about what I thought would go on within the series, boys playing baseball and great deal of slice of life.
What surprised me was the fact I would actually like it, it turned my impression that I wouldn't like it on it's head.
I'm no real fan of Baseball, I won't mind playing it but I honestly can't stand watching it. I think watching it is boring as hell but this series somehow managed to get me interested in it(within the realms of anime, still hate watching it in RL but in anime it somehow works, strange I know)

Sengoku Basara - Pretty sure the PV for this series painted a very different picture than the one I got after watching the 1st ep of this series.

For a few manga titles

Rosario+Vampire - This was before I even bothered to take up reading it but I totally thought it was some melodramatic wangst nonsense shoujo with vampires and well anyone that has read it knows about that one

Carat! - ahhhh yes Carat! I just didn't know it would be so damn funny!
It took my first impression(after the first 10 or so pages) that it would be a tale about magical girls doing magical girl stuff but that agian isn't quite the case. Sure it's got magical girls doing magical girl things but with added (hilarious)twists.
My favorite take on magical girls

There are more manga titles that I probably had a wrong first impression of(just too lazy to list them all-__-)
Titles were I dig around for info and it might be misleading or I jump straight in and think whatever just going by it's title and what it's categorized as.
With anime I tend to do alot more digging around(but not always)as it takes up a whole lot more of my time than reading manga does.

[Skane]

The most obvious one would easily be Suzumiya Haruhi no Yuustu(SHnY). Not having read the source material before, I thought I was being massively trolled by the first episode.

Another one would be Futakoi Alternative. The initial impression was that it was going to be comedic harem hi-jinks. Instead, it was about MASSIVE character development and self-discovery/realisation (with moe twins, cyborg Yakuza, giant squids, biker gangs, etc...(it makes sense in context, trust me)). It's still the number one anime to me.

Rentarou: "Lately, I've been having a recurring dream. It is a totally random dream, about arranged marriages, secret societies, squid monsters and biplanes. It'd be tiresome to explain it all logically but... Well, even though it's like that, if I were to try and sum it up in one phrase... It's probably a story about love."

Do yourself a favour and watch it.

ADD] Oh, and mecha-bulldozers.

Cheers.

[GreatTeacherKen]

^^ I'll second Futakoi Alternative. I thought it was going to be a comedic action series.

It turned into-- well I'm not really sure what to call it, since it has so many elements from different genres that it might as well be its own genre.

Still, it's a good series and I don't really know of a series quite like it.

[Hooves]

Madoka Magica - Same reason as everyone else.

Asarotte no Omocha! - First episode with all this talk about "sucking" made me think this would be heavy, and I mean very heavy on fan-service like Highschool of the Dead. But was proven wrong.

Okami Kakushi - The ending of the first episode reminds me of a certain anime.

School Days- I thought this would be your average high-school anime with your main character getting a good harem and all... Then I started face-palming when Makoto's intent went into action.

Hanasaku Iroha - First two episodes made me think this was going to be very heavy on drama and would stay that way through the entire series. Then episode 3 and its humor ruined that image o.o?

[Rozen Van Kaizer]

I Third Futakoi Alternative and I'm very happy another person ranked it #1 in their list too. In-fact, each of the first 5 episodes will leave you a very different impression than the other, you'll start to think they are from different animes.... each. One episode is chaotic, another very "High", and another very comedic or cuteness and etc. The magic is how the author will pull all these insanity together and in the end gives you (as Skane said) very AWESOME character development and intense action.

Oh and yes this is one of the only anime that will give you a clear-right-to-your-face Harem Ending.

Kimi ga Nozomu Eien - I had to admit, I did not expect that... Quite honestly I was one of those people who didn't accept the ending. But after really understanding how they all went through I had to say that was beautiful.

Ga-Rei (Anime) - Seriously!!? Saw the anime before knowing the manga ever existed so you guys should know how I got fked right there.

Haruhi First Season Didn't like how they introduced the series with that weird episode, but second episode pulled me in so fast.

Haruhi Second Season Cmon people you gotta admit how crazy the first episode was. Really had high hopes for it specially after watching the first season and with all the fans anticipating for this sequel. Then they gave us the "8 episodes"...... and a last episode giving us a message the we didn't go anywhere since the first season.

[Xellos-_^]

first ep of Nanoha threw me off. Didn't go back till a year later when everyone keep telling me how good it is.

[Bonta Kun]

Ohh I have to give a nod to Futakoi Alternative(how could I ever forget that one?!) as well.
Man thats one crazy show, leaves you thinking WTF? but also WOW! as well.

This would have been a series where it's weird and wonderful would have been my favorite part but wasn't the case here, I loved the development of the main characters, particularly the twins. I didn't expect the later eps to turn out that way.

Damn been years since I watched it, now I got the urge to watch it again~

[Skane]

I rewatched it after reading this thread, and it is still AWESOME.

The following extract is my favourite scene from the series.

Spoiler for Spoiler For Episode 07:

One thing I like about Futakoi Alternative is that A LOT of the revelations are implied/subtle, not directly stated/shown/said. You have to really pay attention to the context. The payoff for catching it is, in my personal opinion, Literature Nirvana.

This show is a GOLDMINE for people who like to analyse their stories.

Cheers.