Besides this being Atlus' first 'HD' game, they're really touting it to be a game for adults. The trailers visibly show two of the characters having sex, for example, but the story behind it is the kind of complicated thing that 'adults' can get into.

Katherine (the brunette with the glasses) is supposedly an old friend of the main character--not necessarily his girlfriend from what I've read, but he did recently get her pregnant. Meanwhile, he meets Catherine (the blonde one) and has a kind of one-night stand with her, but then she threatens to kill him (or something) if he ever cheats on her. That's about as much as I know about the story behind the game, outside of the visuals seen in the few trailers.

And yeah, they haven't talked at all about the game mechanics, but it seems pretty clear that it is story-driven and you'll be making choices. One thing that worries me is the logo they use for the game, with a silhouette of Catherine split in half with the 'male' symbol on one side and the 'female' symbol on the other. One anime trope I can't stand is crossdressing, or 'traps', for no reason other than to just have it. The Persona games did this, and this is a game specifically from the team within Atlus that made Persona, so I wouldn't put it past them.

If it turns out that Catherine is/was a man then I will not buy this game.

In any case, if you've played enough of Persona 3 Portable then you've seen the main character of Catherine already.