Well nobody seems to like 4th Edition. Did IWD2 use 3 or 3.5? Either way, I never cared for those two. Too many choices, too many skills, etc. Got in the way of the story IMO.
IWD 2 used a slightly modified version of 3.0 iirc.
Anyways, I don't have a problem with them sticking to 2nd Edition for the BG games. I imagine it would probably be too much of an overhaul to change that. I'm satisfied with them importing the extra character classes from BG2 to BG.
I REALLY hope this is successful enough for them to do the same thing with IWD 1&2 and Planescape.