Some of this, I'm sure, comes down to matters of personal taste. I know some players and GMs who love really intricate, detailed rules systems that try to make social interactions, combat, and exploration all as life-like as possible...
Well, I mostly play and run GURPS 4e, which is fairly detailed. We do not try to drag in rules that aren't applicable to a setting or genre, but there is quite a lot of material for D&D-style fantasy, and for semi-historical settings. Replying with that in mind:
Erratic number goals. Do you want to roll high on this check, low on another? Are some skills percentile while others on a d20 or a d6?
All rolls to see if you can do something are 3d6 roll low. You want to roll high for damage, and you want the GM to roll high for social reactions (players don't need to roll those). This works fine; the classes of roll are clearly separate.
Charts. I'm not talking about a handy list of what you get each time you level or what spells you can select. I'm talking about each and every combat or skill challenge to get out random charts, roll percentage dice or whatever to see what happens.
Occasionally you need to look up a skill's description, but there aren't any more charts. There are charts for critical hits and misses in combat, but you only need them when you've rolled a critical, and they're one more 3d6 roll.
Multiple maths used in each roll. Did you hit? Compare the target number to your die roll. Then divide by another number to see how many ranks of success. Then add to a feature of your weapon. Then subtract the opponent's armor rating compared to the AP rating of the weapon.
Basically, roll against your skill. If you want to attack specific hit locations, or make the opponent's defence roll harder, there are subtractions to your skill, but those are optional, and controlled by the player. Ranged combat requires asking the GM for for the range penalty, and if you're using a weapon that fires multiple shots, a very small division to find out how many shots hit. Roll your damage, subtract armour, apply it to the opponent.
Hidden descriptions. "The monster is undead and has all the undead traits." Then you look up undead traits to see immune to cold, negative energy, poison, charm, sleep, etc. Just put all of that in the monster description so I don't have to look it up for every undead creature every fight. Or every plant, or demon, or whatever. How am I supposed to remember this stuff?
Reasonable in the era of PDFs. In the print-only times, keeping down the page count was important.
Complexity that obscures the nature of what is going on. Example: Multiple roll successes before failure(s) that hide the true odds of success/failure.
Some game designers seem to have a "cargo cult" view of mechanics. "This worked great for me in (really cool campaign I ran years ago), so all my games should work that way.
Basically my two axes of complexity boil down to "how hard is this game going to be for me to teach at the table" and "how hard is this game going to be for me to run at the table".
Whereas we aren't really into system-hopping. Different players in the group play with different default levels of detail, and nothing breaks. We had a campaign run by our least technically accomplished player in 2019; she just ran it as "human being with skill rolls", much in the style of CoC.