The controls - trying to emulate button/circlepad controls on mobile doesn't work. Use a a control scheme that works well with a touchscreen.
Monetisation - That microtransaction jungle....
Progression - give me a reason to keep playing, as others have said lots of mobile games become boring fast, and if score tag or get 3 stars is all the progression your game has it's no wonder why. Evolve the mechanics, give me a storry, just do something.
That said not all mobile games are bad, it's just hard to find the good ones.