Some things to make you stop and stare ...

[Fjordmonkey]

A lot of good ones in this thread.

For me it's definately the cockpit shadows, which more than once have led me to continually roll my aircraft to see how the shadows play against the rest of the cockpit (has led to not just a few crashes, as I'm quickly running out of altitude and options while mesmerized by the shadows).

That, and the feeling of flying low over the terrain, hugging every little hill, valley and bump in the terrain. Especially fun to do in a 109 when you're being chased by a few very angry Spitfires. Just pay attention to your altitude and the terrain, lest you become PART of the terrain.

Absolute best part, though: Sunset in hazy and partially clouded weather. Just makes me want to fly around watching it for hours on end.