There's really only been one rules change request I've consistently refused (generally I operate fairly close to RAW/RAI anyway note in 4E and 5E, not so in earlier editions).
Critical Fumbles - I've players ask for critical fumbles a ton over the years, and because I've had experience with them, and it's been purely negative, I've always said no, and will continue to say no. Rolling a 1 is too common for something dreadful to happen, and I have never, in my life see a critical fumble table that didn't either turn the game into straight-up Three Stooges-type slapstick, or some kind of weird gory festival of broken bones and so on. Or I guess, into the episode of Supernatural with the Rabbit's Foot, which is like, hilarious for that episode, but I don't want D&D to permanently be like that. With critical fumbles it pretty much is. A combat might very easily feature 10+ attack rolls a round, maybe twice that sometimes. That basically means half to most of
all rounds include a critical fumble with a four-five person party and a few monsters.
Critical hit tables - For similar reasons. I conceptually liked the idea back in the '90s, but every implementation I saw was total arse, like simultaneously too specific and repetitive, and not specific enough.
EDIT oh and:
Rolling for HP - You think you want it, but you don't.
Every time people are like "Oh rolling for HP is cool" (except, notably, the one guy who was consistently burned by it back in 2E), "Can we roll for HP Ruin?" "No. You know what'll happen." "But this time we'll just live with it instead of moping because we kept rolling really badly and now have less HP than the Wizard!" "No.".
It's just a great way to make so your character might be absolutely unplayablely bad (and yes, a frontliner like a Fighter or Barb rolling 1-3 on HP consistently is unplayable levels of bad, esp. if the did put CON secondary). Yeah everyone dreams that they'll just roll 12s but that doesn't happen.
we have a requirement to resolve your turn in 30 seconds
Christ, I think most of the people I play with would literally have a heart attack and die if put under than kind of time pressure. 30 seconds is barely long to even describe the effects of most spells, let alone to resolve stuff like multiple attacks and grapples and so on.