So, thoughts so far, 3/4 hours in.
The opening
Elizabeth
Show restraint with lockpicks, even if you get the extra 5 from pre-order.
The rest
It's Bioshock in the sky, down the to plasmids, random tapes, eating food, vending machines(same VA too), guns. Just name changes on these things instead. The rails is the main major addition, along with Elizabeth. Also no hacking so far, not sure if it'll be in ever actually.
edit:
Bugs
Yeah their miles of code contains a few odd bugs here n there, nothing majorly game breaking though one is an annoyance. First is a visual bug, when bodies disappear after being attacked with Murder of Crows, the blood spatter on them remains where the corpse once was. The other is that some users, myself included, aren't receiving any steam achievements. I had disabled Steam Cloud already, not sure if that's a cause or something I'll be glad to have removed later if it turns out the Steamworks implementation (thier last game was GFWL, so they're a bit new to it) isn't fully up to scratch and saves go vamoosh.
So yeah as a precaution I'd maybe disable Steam Cloud.