Anyway, to summarize: it's a new class (incidentally the one that arguably introduced the least amount of novelties), zero new races, a bunch of graphical tweaks and a new area BUT with no raise to level cap.
New class, new spells - as far as class novelties: wild magic, and metamagic. new area, and apparently some updates to underdark (how extensive those are I don't know). New lignting improvement, which does enhance the look of a game quite a bit. Extra weapon skill unlocked with proficiency depending on the quality of the weapon.
P.S. also, not sure what they changed about how they process updates but it's taking forever for me on Steam, while in the past I was always done in mere minutes.
Considering the graphical improvements it is possible that quite a few core files get changed. GOG patch is 76GB. Outside having to download the content (going 13mb/s I am 50% done) it is possible that rather that just adding content there will be a lot of modifying/overriding content. Swen also recommended checking integrity after it is done, to ensure all patched correctly. It is possible that the PC will have to do a lot of heavy work when patching the game with the changes.