I wish there was a rule from the start when designing KDE4. Whenever you really really need to not use one of the already very nice default Oxygen style buttons, and can find no way around actually creating your very own custom look button, please, PLEASE do it all the way and make sure it has unique, clear looks for the normal, hover and pressed down state.
Looking at the 4.2 desktop now I see so many widget elements breaking this rule. The most obvious (and possibly also the most important) is the start button – which has only one hover picture. When you press it you get no feedback at all, and the mental image I get every time I click that button is hitting my fist against a brick wall (why? it’s hard to explain, it just feels so hard and unmovable without the proper visual feedback).
Actually, when I think about it, this is actually the same for _all_ the buttons I have on my bottom panel. Including the systray expander button, which actually doesn’t even have a hover state. The task list is an exception, that one is actually doing it right. (Not that that one gives me a lot of feedback either with the all too subtle color change it has, but that’s really a problem with the current style, and not kde by itself).
A quick end note, to be honest, this was actually the kind of things I set out to fix when I slowly started KDE development some time back with ui improvements to Ark. I eventually got stuck maintaining ark instead, and these small annoyances that I have been thinking about for way too long, had to wait in the back of the queue while Ark was getting fixed from the bruised and bad condition that I found it in.