Being able to functionally play a 3.5 character in Baldur's Gate is a fascinating idea.
Minmaxers and powergamers would have a field day with it, since the system is so much more open than 2e's multi/dual classing system. It also seems like something that would break extraordinarily easily, either the difficulty curve of the game, or the actual game itself, without a lot of testing, and once things other than EET and IWD-In-EET are thrown into the mix, all bets are off.
Still, I remain incredibly excited by the idea and possibilities that come from shoving all of these games together.
EDIT: It seems like the class system would work more like PnP than NWN's, with all of the class-sepecific restrictions in place. That does make me wonder though, how are monk's AC bonuses calculated? Does their AC improve based on their wisdom in this hybrid system? Are existing BG kits compatible with this system? Because I can very easily see Monk/Cleric and other similar combinations as being especially powerful under this, just like they were in PnP and NWN.