Torsten -- I don't agree. I've been in Malta the last week and used Marble on a daily basis. It works great for me in its current state.
CPU and memory usage are fine on my N900. There was another user reporting a (probably memory related) problem that lead to laggy behavior of the UI. I guess it happens when the remaining memory of the N900 is small. Marble needs roughly 50 MB RAM. We can trim it down a little bit more in the future, but there are not too many possibilities.
Not sure about font sizes. I guess that refers to the fonts in the map of the OpenStreetMap theme? We're just rendering the OSM tiles there, so it's more the size/resolution ratio of the N900 to blame. Of course vector maps could solve that, but that's a feature for the distant future. Or do you mean the fonts of the vector based maps?
Which buttons lack visual feedback? The ones in the menus are standard Maemo buttons. If they don't provide visual feedback, I can only think of memory shortage (laggy UI) that leads to this. Or do you mean the four buttons on the bottom of the screen?
Please be more specific and report bugs at http://bugs.kde.org. Thanks.