Everything. With new classes, kits, races,... and all the other engine changes it's just not BG1 anymore.
Races? The only new class I've used has been the sorceror and I rarely use kits as I don't really like any of them. Considering all the stuff that I've got changed back to the BG1 method through mods, I'm not entirely sure just how much is actually different. Most of the rules have been changed back, I had the BG2 spells removed, weapon proficiencies are grouped instead of individual. Walking speed is BG1 level so ranged weapons are just as deadly as they were before.
Lol, you did all that instead of just play with WS mod?