Obnoxiously complex and layered skill trees.
Your character is leveling up, your weapons are leveling up, your clothing is leveling up, your vehicle and/or mount is leveling up, you can choose to level up your character in 20 different ways, and within each one of those ways there's abilities you can choose to unlock, and each of those abilities has different perks you can apply to them. Your weapons level up, but you can also add augments and attachments and customizations and upgrade the bullet type, etc.. etc.. Same goes for clothes and cars/mounts with add-ons and accessories and whatnot.
For me, it just gets way too complex to the point that I just completely lose interest in leveling, and in-turn lose interest in progressing further in the game altogether.
I think that shit needs to be reeled in and progression should be simplified and more impactful. Right now the difference between being level, say, 15 and level 20 is practically nothing, and instead you're given all of these little things I described above to make it feel like you're changing something substantial when you're really not.