No multiclass, but less need for it. Character building is both more and less open to experimentation.
All the stats do the same thing to every class, so dump stats are determined by role rather than by class.
All characters can wear any armor they want, no class restrictions, but armor has a speed penalty.
Skills only receive a small beginning boost based on class, so it makes building different skill builds much easier.
More feats are universal, which also means class is less restrictive than normal.
Levels offer set bonuses to accuracy, which means that when you level up you feel it instantly.
Classes in general are more distinct from each other, but lack of multi-class limits build variety.
All of this leads to subtle variations in characters, where your gear and playstyle are just as if not more important than your build well into the middle of the game. As the game progresses, talents start to offer a big difference in viability of playstyles and builds become both more effective and more static.
That said, party builds are significantly more unique than the IE games (if you’re willing to use non-story characters). No hard counters means that many more builds are effective. You could run a three barbarian, three wizard party with little problems and it would play very differently from a three paladin, cleric, rogue, mage party. Unique parties can provide the game-breaking fun of a ridiculous multi, and provide a unique tactical experience.
It does have a few kinks to work out, and has significantly less content than either BG game, but I wholeheartedly recommend it.