IWD was a stop-gap between BG1 and BG2 (although undeniably a game in its own right - and a good game at that).
Hence it is BG2 with the new classes - IWD just has the standard BG1 ones. HoW is just a mission disk, like ToSC was to BG1.
BG2 with its classes, then, is a different game entirely. Throne of Bhaal will be to BG2 as HoW is to IWD and ToSC was to BG1.
Enough acronyms for ya?