Hmmm... Overall, a lot. If you looked at different sections, it would vary.
Races: Complete Revision
Classes: Most are Revised, Some Dropped, Some Added
Skills: A Few Changes (mostly added detail and a few new ones, Opposed Rolls rather than set DCs)
Feats: A Bit (added prereqs to redefine power-levels, a bunch of new ones, etc.)
Equipment: Changed per Region, New Weapons, New Armor, etc.
Combat: A Bit (W&V, Facting Rules, Defense Rolls, etc.)
Magic: Most Aspects Revised (Elemental-based Spell List for Sorcerers, development of "Higher Arcana" spells that while leveled 1-9 are "PClass Only", Greater Difficulty and Cost to Item Creation with a degree of failure, etc.).
Monsters: Mostly unchanged stat-wise, but require the proper "feel" to be used. If used, then given a suitable niche within the world and its history. Also, encounters with magical beasts and monsters aren't the norm, but rather the exception.