We know the game has been in development since mid-2011 and Ubisoft mentioned in their meeting earlier that this game has a 'new team', which makes me think that this game has been in development parallel to, and independent of Assassin's Creed III.
The game's writer is the same as the one for Revelations, and if there's one thing I can give that game, it had the best writing in the series. Ubisoft has promised proper open-world assassination missions will be back, hand-held linear missions will be less, and the rest of the game sounds like a Sid Meier's Pirates 2. On paper at least, this game appears to be everything Assassin's Creed III wasn't.
I don't see what's so bad with it being a yearly franchise. It need not directly mean a deterioration in quality, especially not for a gigantic company like Ubisoft and UbiMon, which must be the world's largest single game studio. Despite my gripes with the series, I find it impressive that they're able to create such laboured worlds and consistent, story-based 20-40 hour experiences on a yearly basis. I'm actually kind of glad we get to see so many ACs in different settings so soon - the alternative would be what, waiting five years drawing straws to see which setting they'll pick next?
And to be honest, what could Ubisoft possibly improve upon in an AC game, given more time? Less bugs granted, but if there are any significant flaws in the AC franchise, they are a conceptual, design level, not something an extra year of polish is going to fix. Hell, it'll probably get some wise guy to include five more completely pointless side-games (ooh let's add a bomb crafting mechanic! Oooh let's have a farmville in the game!)