The user interface doesn't provide feedback fast enough in a lot of cases. I find myself clicking twice on a button because I think I must not have clicked it the first time. Responsiveness is also an issue when there are a number of steps to perform an action -- if I'm reviewing my activities and think "Ah, I did that power-up, I should enter it" -- I've got several steps -- enter the power-ups mode, maybe scroll to the right one, view it, click on I did it (maybe twice!) then OK the dialog about points gained. That many steps is a bit of a nuisance, but it is made worse because the interface is somewhat slow.