I don't think that there's any one set way to make a hit anime, but I think that there are several common (not universal, but common) strands that you find through out the hit animes of the last few years:
1. High production values. In other words, the animation quality is top-notch and the art style is very polished. The quality of the visuals will have an impact on how likely an anime is to reach a 'hit' status. I've seen animes that I could tell, just from the lack of polished artwork and top-notch animation, that it wouldn't be a hit. I've also seen breathtakingly beautiful anime, at a visual level, that I could tell had a chance to be a hit because of the visuals.
2. Strong Beginning. The importance of this is somewhat understated, I think. It's a huge help to an anime's success if it gets off to a very strong start that really grabs in the viewer with the first episode. Code Geass, The Melancholy of Haruhi Suzumiya, and Bakemonogatari are very clear examples of this. In each case, the anime gives you an eye-catching first episode that features a lot of plot activity and/or zany events. Outside of moe or slice of life-centric animes (where character appeal is probably more important than plot handling), slow starts can kill an anime's hit potential right off the bat. A good rule of thumb is the three episode rule. If an anime doesn't leave a strong positive impression on a lot of people after three episodes, then it's probably not going to be a hit anime.
3. Good Comedic Moments. The hit anime usually needs to have great comedic moments. Now, some of the jokes can fall flat, but at least some of the jokes have to leave most of the audience laughing loudly in response. And there has to be some humor. This can certainly include fanservice, but it doesn't have to be fanservice. In some cases, witty repertoire can provide this (as seen in Bakemonogatari and Durarara!!). The one exception here might be Gundam and Gundam-esque mecha anime. Those can generally get away with being very serious almost all the time, if not all the time.
4. A Key Area of Strength. The hit anime can be very flawed, but it usually needs to provide a certain quality that leaves it comparing favorably to other animes (at least for people that appreciate that quality). By that, I mean, it usually needs to be seen as the best of something (the best of moe, the best of slice of life, the best of shounen, the best of harem, etc...), or it needs to be viewed as offering an unique combination of certain anime flavors (Lyrical Nanoha with magical girls fighting mecha-style, Code Geass with a veritable potpourri of anime elements, Higurashi with a mixture of slasher horror fare and moe, etc...).
Now, by "hit anime", I'm going by animes that sell well on DVDs. Not animes that are simply talked about a lot on-line.
Getting talked about a lot on-line is much more a matter of sheer luck, really. If your anime includes a short scene that catches a person's fancy, and gets turned into a meme, you've just about guaranteed that the anime will be talked about a lot on-line. But memes alone won't move DVDs or merchandise.
Last edited by Triple_R; 2010-11-28 at 20:59.