Well, I played Azorius in the midnight prerelease on Friday, and I can say the following:
Scavenge really doesn't do much. Trust me, three of my four opponents were running Golgari decks.
Unleash's drawback never really seemed to be an issue. Honestly, they could have easily just printed them all with the extra 1/1 and "___ cannot block." and it really wouldn't have made a difference.
Overload is pretty neat. All I really saw were the ones I had pulled from my packs, and most of those had red mana in their cost.
Detain is hilarious in action. Of course, my deckbuilding skills are bad, so I still failed horribly.
And... well, the only undefeated deck was Selesnya, so... yeah, populate is obviously the go-to mechanic this set.
As for Vraska... I can't really say much, seeing as the closest anyone came to seeing her that night was me pulling the token she produces.
In terms of favorite cards... you might laugh, but I'm honestly quite fond of Swift Justice. +1/+0, first strike and lifelink on a one-drop white instant is quite effective. A secondary favorite would be Righteous Authority... even though Maro + card draw really feels like it should've been in Simic instead.
As for my W/L record... if I ever got to see Righteous Authority for more than one turn, I won. Of course, that never happened more than once a match, so I ended up coming in last(again, because I'm terrible at building decks). Had I been thinking, I would've taken advantage of the Steam Vents and two Izzet gates I pulled and just ran Patriot colors. Probably would've done better that way, actually.
Hmm, well it's definitely reminiscent of of the old Ravnica block. That new Planeswalker Vraska looks awesome. I can see her being big in BUG. The only new keyword abilities I was disappointed in were Scavenge and Detain, Populate is by far my favorite. I think it's gonna be an excellent Standard environment with RTR.