Lacking in feedback. You're basically just waving your weapon around and watch damage numbers pop up.
It's why more action focused games include things like enemies flinching, animation locks, stagger mechanics, etc, to convey impact, so you don't feel like you're just making your attacks vaguely collide with a hitbox while shuffling around.
Guild Wars 2 has exactly that problem, because everyone is so mobile in that game and attacks have little to no commitment it really feels like you're just waving a pool noodle around. But it's not exclusive to GW2, a lot of tab target mmos suffer from it.
