If I'm remembering things correctly, the Miniatures Handbook invented the concept of the swift (and immediate?) action.Yikes! Had to look back - 3.5 PHB was released in July 2003, Miniatures Handbook came out in October 2003 - so you are correct it's 3.5. I bought the plastic minis left, right and center for my D&D games to replace my metal minis, but never played the Minis game.
I was an early adopter of 3.0, but it was about 6 months to a year before I switched to 3.5. D&D was my life back then, and I had invested a lot of money into 3.0. I disliked the idea of buying the new core books, even though I was buying the other 3.5 books as they came out. I sort of gave up and switch core books when the conversion guide was ~40 pages.
Seemed like a useful thing to have included in the 3.5 Players Handbook, but instead they had to keep explaining it in every new splatbook that used it - although it is in the 3.5 SRD, so maybe I am misremembering?
3.5 was a strange mixture of helpful changes (beefing up the Ranger, moving a few key class abilities of other classes to 2nd level to make 1-level dipping less attractive), neutral changes (horses now take up 2x2 space instead of 2x1) and changes that arguably made things worse (I seem to remember changes to the Charge description and/or the Ride-By Attack feat meant that the latter didn't actually work as written).
I bought the 3.5 Players Handbook as soon as it came out, but couldn't see any reason to buy the other two books - although I eventually gave in and did so, since there were enough tiny changes that the cumulative effect eventually became too much for me to ignore.
I suppose it was good practice for switching to Pathfinder - I am still occasionally finding changes to 3.5 there!