I'm really looking forward to playing as Pharah and Symmetra.
Pharah I like because it probably suits my play-style the best. I've always liked using rockets in games, and being able to jetpack straight up into the air will definitely be useful.
Symmetra seems to have it all. Teleporters are great, mini slowing-sentries will be handy, shielding allies will be valuable. I suspect she'll be pretty good in combat too. She doesn't have a lot of health, but her charge shot goes through shields and enemies, meaning she could potentially kill an entrenched Bastion or shielded Reinhardt (and the people hiding behind them) a lot easier than other characters.
I think Winston might be fun too, just because he's a tank that actually has mobility.
The characters I'm not as interested in are:
Torbjorn: He just looks boring. Trying to keep a sentry up isn't really my cup of tea.
Widowmaker: I'm a terrible sniper, and don't particularly like being sniped. Probably will barely touch her.
Reinhardt: He's powerful, but he has almost no ranged attacks, and no mobility skills to get enemies above him.
Bastion: He's good(and basically a transformer, which is awesome), but I don't think I'd be good and choosing places(and times) to entrench at.