Voadam
Legend
Gotcha.Spell-like abilities, mostly. Check out what a 3.0 pit fiend can do in that regard versus what a 3.5 pit fiend can do; it's rather stark.
Admittedly, this is a very niche area, and one that doesn't apply to a lot of monsters, but for some reason it's always stood out very strongly in my mind. The 3.0 pit fiend can identify targets worthy of corruption via detect good, lure them into sin with suggestion, and defile holy sites with unhallow, etc. The 3.5 pit fiend is basically just a combat machine, though it at least kept the 1/year wish.
3.0's glass cannon outsider spell like ability platforms versus the 3.5 beefed up combat stat bruiser ones who can fight better at their CR class but have stripped down magic.
3.5 struck me the other way with a little more descriptions in the SRD (in that all monsters had one) and with more flavorful descriptions.
From the example pit fiends you linked to:
3.0 ". . ."
3.5 "A pit fiend often wraps its wings around itself like a grotesque cloak, and appears wreathed in flames."
For the actual MMs they are mostly the same brief descriptions but 3.5 has a little more.
3.0
3.5: