A lot of simplification is good, when 3e came out with its unified d20 mechanic, that was great. Now it was the same mechanic for hitting something, rolling skill checks, and making saves. No more separate attack rolls, roll under for skills and roll over for saves, and no more percentile skill checks for thieves. I ever minded thac0 but I also get why they changed it to ascending (something they wanted to do for 2e, but didn't because they didn't want to invalidate 1e, I guess conversion documents weren't a thing back then)