When a new D&D edition comes out, every gaming forum gets flooded by "oh noes I need to buy all my books again". The passage from D&D 3e to 3.5e is somewhat special in this regard because most core things that aren't in the 3.5 SRD have not been changed too much (or at all) from the previous books. (While some others like psions have heavily changed and some variant rules did not even exist in 3.0, so I'll be talking about the three core books only.)
If a DM and his players owning 3.0 core books wished to play 3.5 and replaced the crunch in the books with the crunch in the 3.5 SRD they won't be playing 3.5e yet for some things wuold still differ.
E.g. the 3.5e Mind Flayer is not statted in the SRD and the wannabe 3.5 DM using the 3.0 one would be using a different monster (the same is true for a list of D&D-specific monsters: beholders, gauths, carrion crawlers, displacer beasts, githyanki, githzerai, kuo-toas, slaadi, umber hulks, and yuan-tis.)
What crunch will I completely miss (e.g. the cosmology chapter) or get it wrong when playing 3.5e with every 3.5e book but 3.0e three core books instead of 3.5e ones?
You can and should include the monster list so that the answer is useful to everyone without referencing the question. Any reference to 3.5 material that's in the core books now like the cosmology chapter will be great