Adventures that used tactics relying upon old spell mechanics (all day buffs for NPCs, endure elements for specific elements, polymorph, etc.), or monsters that changed power levels and CRs in the revision.
Monsters won't be tough to upgrade for use in a 3.5 campaign or new product, DR should convert easily and they might be off a feat or two or some skill points but no big deal for use in game.
spells that were based off of old versions of spells (BOEM mark spells) are impacted.
Most splatbooks are still useable, feats, prestige classes and many spells are going to be valid in both 3.5 and 3.0