here's mine...
1. Scale - as mentioned in some of the previous posts, scale is very important and easily overlooked - particularly in relation to fractal textures and haze settings. I always have a human figure .obj to terragen scale ready to drop into a scene - immediately gives an indication of a real sense of scale.
2. Camera animations - I often see a great looking scene ruined by a camera move that's impossible in the real world. In fact I'm at work looking at one right now :-) Try to think of the physics of the camera - is it possible? Imagine your camera is on a crane or a dolly and try and think the limitations they may impose on a camera move. I animate my camera in Maya, and I have a script that tells me how fast my camera is moving in miles or kilometres per hour - that way I can keep the animation real and is one less thing that might "take you out of the scene".
3. One handy thing I found (and this might just apply to Maya) is if you import an object from Maya and want to texture it using procedural shaders in Terragen - you need to delete its UVs.
