>Magic can do anything and everything.
Honestly the biggest problem with DnD/Pathfinder, although 5e reigned it in to some extent. Still, when magic exists in a system, it shouldn't be so versatile as to possibly replace everything.
If magic itself IS that versatile, then write in something that restricts casters to picking one school or specialization. Say their magical energy adjusts to whatever they specialize in and makes other magic harder (or impossible) to perform.
>Only "divine" magic can do healing.
Yeah, fuck this. Fucking stupid, can't even articulate why, it just is.
>Multiple attack growth
Another thing 5e largely reigned in, but I always hated in 3.5 when your BaB got high enough you were doing 3 or 4 attacks per turn, because it make "full round attack" the only viable option for martials 99% of the time, and there was alot of math and unnecessary dice rolling involved.
>Mechanics where one player is playing "an entirely different game" within the game.
See Deckers in Shadowrun. Every time they attempt to hack something, they have to go into cyberspace to do these videogame-style hacking battles. Meanwhile the game grinds to a halt because the rest of the party is just standing there in the real world with their thumbs up their ass unable to help or be involved at all.
>Experience points
Self Explanitory, just give level ups after big plot arcs of story moments.
>Negative Levels
Also self explanitory, also a shit penalty for deaths as it creates uneven parties.
>Revival mechanics
Short of the intervention of a god or time-travel, death should mean dead. I'm not a fan of hyper-lethal campaigns, but I DO like tension, and revival mechanics just suck ALL the tension out of a game.
>Summoner mechanics
I like summing as a mechanic, but in every system I've ever played, it turns into a complicated glorified pokemon sim, with lots of booking and stat blocks involved. Also infinite mook parade, tedious.