(I think you are asking about development so I will answer as such, if I missed your point let me know..)

By Hardware capabilities.

Theoretically I could write a QT app that uses hardware keys to navigate things or drive a car.

Problem: Half our MeeGo devices don't have a hardware keyboard! They can't play my game! (I believe this actually happened a bit in Androids infancy when the G1 was the only device. When the MyTouch came around I seem to recall people complaining that hardware keyboards were required to use the app and they didn't have one.)

Or, QT is also on my desktop and as dov said earlier: I could code my app to be a Desktop app in QT with a Desktop-style UI (or require left, right and middle mouse buttons for example) and then someone tries to port it to MeeGo or Symbian (because it's QT). Unfortunately the UI will be unintuitive at best, unusable at worse on the smaller devices.

Also, there is likely to be at least some internal differences between Symbian and MeeGo.. so if I code my QT app to rely on something that only exists within Symbian, then porting it over to MeeGo is not a straightforward process. It will need to be fixed/worked around to get it to work.

Etc.

QT (or any platform that I've seen) is not 100% "Code once, deploy anywhere" when you actually talk about 'usability'. Sure, if it's in QT we can likely make it run on our devices... but whether we can use the App or not is a different story.