2e: Thac0. It’s so obvious, in nearly every facet of life that has any math at all, that the simplest possible situation is counting up and adding relatively small numbers. It’s hog wild to me that someone said, “let’s completely redesign how hitting things works, mathematically, but let’s focus on subtraction, with a scale that goes from a basically random positive number down to a basically random negative value!
3.5: (other than most of it) Bard songs taking you’re whole entire action to start. Just let characters be effective at the thing they’re thematically defined by! Really, all the weird fiddly restrictions and limitations that were mostly just frustrating at a gameplay level, without actually adding anything thematically.
4e: The refusal until Essentials to let a class have two power sources, meaning a wholly mundane Ranger, and making it impossible to have build options that make the other martial classes have a touch of magic.
Also the vancian-esque single use of a given power. Boggled my mind that we didn’t have power slots and known powers, with wizards getting to swap out powers during rests.
The math. Hated it less than 3.5’s math, but sweet baby Cthulhu I hate a numerical power scale so wide that the two ends simply cannot exist on the same board.
The weird decision to make it take 4 feats to fully mix power sets of two classes. It should have taken 1 feat. You MC, gain a skill and a minor version of a defining feature of that class, and you can now take powers from either class. Done.
5e: We houserule and homebrew 5e more than any past Edition, so it’s harder to remember the things I don’t like.
Definitely having to choose between ASI and feats, while also having a relatively low pointbuy (I think it’s 25 pts?). I know that you can play with a character with nothing higher than 14, but it feels off, and contributes to people feeling (even if erroneously) that they need to take ASIs rather than feats.
The relative lack of advice or rules for followers, home base, organization, magic item costs that make sense and are usable in worlds where magic items that aren’t rare or higher…aren’t rare.
The low number of skills many classes get.
no way to gain more skills other than new class feature, feat, or multiclass.
Making so many downtime activities not involve any variability that could lend to rolling dice.
Backslding on inclusive/sensitive depictions of the world, in favor of appeasing a specific part of the old guard, leading to things like Volo’s descriptions of orcs and Gnolls,
The Sundering.