What game mechanics when introduced were absolutely hated?

[Garthanos]

I just liked it as a player, and never really felt like the overhead on the GM side was too bad. And I'm mostly thinking of damage on a miss due to a special power.

it makes the character feel like they are able to do something special instead of being a feature of the encounter balance like in 13a.

 

[aramis erak]

THAC0 was not hated when it came out in 2e. Most of us were already using something very similar by then because it was easier than attack matrix tables, and freed up space on the character sheets to just have a single number. It first showed up in the 1e DMG at the back, so it was a natural progression. And it was an improvement over attack matrix by all but the most purist of grognards.

Many of us hated it from the get go, because it required remembering the flat spots or ignoring the flat spots, and we were good with writing the attack line down the right margin of the Mk1 generic character sheet (a sheet of notebook paper). It also took longer than using the on-sheet lookup.

I was certainly not a grog in 1981, but I disliked THACO from the moment I encountered it.