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One thing to keep in mind is the *relativity* of usability. To say something like ok-cancel vs cancel-ok or panel at top or bottom of desktop, or navigator vs. spatial file manager, etc.. is like saying "Apples are better then oranges because I like, and am used to apples, and this is my first orange" That means, expecially when philosophy and focus in UI are so radically different, that when it comes to GNOME vs. KDE discussions (or mac vs. windows), know your background, and try not to bitch too much unless you really "get" the new thing you're using.
Where good vs. bad comes in is more a matter of consistancy or refinement. You say "This is bad because it doesnt work in the same paradigm as the rest of the system." A good example is in windows XP, go to the default control panel view. I have never, ever seen anything even remotely similar in any other windows component or application for configuration. What that means it its not only jarring, but it also requires a user to "switch gears", to go into a new mode of thinking while using that comonent. This is also why list view for OSX is the worst thing the could have possibly done, mac has alwas been the king of consistant interfaces, but a mix of the spatial and navigator methodologies, no matter how good the implementation, is ALWAS a Bad Thing.
Anyways, all im saying is stop and think about how everything works together. A valid complaint in GNOME for example is the old file selector. It just wasnt anywhere NEAR as useful as it could have been. There was also not any valid reason for this, nothing in the overall usability of gnome to require something simple there. Therefor, it was a valid complaint, and bad. An INVALID complaint about gnome is that it isnt powerful. The whole methodology is to have quick and simple access to everything that most people will use the most, and keep everything that five people will ever use into the far background. This isnt bad, its a choice. To say that all options should be a click away, vs Only the very frequently used options being a click away is saying "We believe that the user should be exposed to everything they can configure, even if it takes longer to find what they want, at least they will know whats possible" vs "We believe that interfaces dont have to be complex, configuration should be grouped logically, and only things the user will change on a regular basis should be in the forefront, as it will make it easier to find what they want to do, and keep them from being overwhelmed." apples and oranges...