Nothing. There's nothing so broken that it will completely demolish play. There are highly effective builds, and some tricky combos that get good results, but nothing so broken that it will completely overshadow the rest of the party (provided they haven't actively tried to gimp themselves).
There just isn't anything as above the curve as a 4e Ranger, or a 3.x CoDzilla.
Side note: There aren't really any BS builds either. Just combos that people think can be fun.
Nothing that breaks the mechanics of the game (besides some elf warlock sorcerer builds), but when a DM retunes encounters to challenge 1 optimized party member, the other players may feel useless.
Say you have a Great Weapon Master Bear Totem/Champion in the party. between levels 5-10 he is putting out ~70 damage a turn and killing 2 things while resisting everyting. Once per short rest he gets a 120 hp nova round.
Add in things like Cleave and Flanking, and you've got a guy telling other players where they have to stand and which guys they aren't allowed to attack yet.
He might ruin the experience for a Polearm Fighter or a Dual-wielder that is only putting out 25-30 a turn and not getting any kills against enemies that have had their hitpoints maximized to account for the Barbarian's damage.
The risk of builds ruining the play experience is MASSIVELY decreased if you use point buy instead of rolling for stats. Then at least no-one can start with 20 in a main stat and feat choices mean actually sacrificing an important ASI. You don't even need to stick to 27, even a 31 point buy is reasonable. And do NOT use the flanking rule.
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