I would've selected your answer as best but you mentioned the plane scene which I found perfectly hilarious as well as down to earth.
You rightfully pointed out that Ajay needed better characterization to make him look more believable as the rockstar.
The climax, it was always supposed to be a happy ending, Salman not forgiving Ajay wouldn't have made sense. But the climax should've been handled better infact a lot better. It came out as very ordinary & dragged in the end.