If there's one things that's slightly off-putting with Reaper it's the flickery and somewhat glitchy GUI. Try docking the Media Explorer and resize it to see what I mean. This flicker only seems to happen with docked windows and I didn't notice it in the OS X version (although that one has its own set of GUI issues).
Also executing some big actions can cause a real flicker fest in Reaper. Not to mention the stuttery playback cursor. Don't get me wrong, I love Reaper, but after trying Studio One (which is buttery smooooth) I really hope they can improve the general smoothness of the GUI in Reaper.