The way that the mouse controls the camera could be a lot more flexible. There's an old-ish planetarium app called Celestia whose camera controls, I think, KSP should imitate closely. There are three main differences between them.
One is that the camera has unlimited movement in all directions around its focus. I'm not quite sure how to explain this, but let's say you're viewing Mars. You hold down the right mouse button to move the camera around, just as in KSP; but if you keep dragging your mouse in a given direction, the camera will continue to revolve around Mars, without interruption, for ever. KSP's camera can only circle once in any direction before it stops and won't move any further.
The second main difference is that Celestia's camera can roll. To do this, you hold down both mouse buttons and move the mouse to the left or right. KSP's camera obviously can't do this. Also, moving the mouse forward or backward with both buttons held down will move Celestia's camera toward or away from its focus. This control is redundant in Celestia, since the mouse wheel does the same thing, but in KSP it could free the mouse wheel up for some other control. (Throttle, maybe? And in the VAB/SPH, it could replace shift+wheel as the in/out control.)
The third main difference is that Celestia's camera pans/tilts when you hold down the left mouse button over an empty space (i.e. a place on the screen where there are no buttons or objects to left-click on). KSP does the same thing when you hold the middle button, which is why I don't think that this difference is as important as the first two, but doing it Celestia's way could free up the middle button for something else. (My suggestion: it should toggle Map View.)