To add to that, cube shaped rooms are really boring to look at (even if you do make a neat puzzle in it). At
least use two different ceiling heights. Breaking up the cube shape is really important - I think "The Second Exodus" is a great example of a boxy room done right (see level 3). There's a hole with a view of the skybox, things in the corners, and a really big module in the center that draws your attention away from the sides.
Shorter = better when it comes to verticality in tests that don't use momentum. Don't make it too cramped, though. Tests should never be 1 or 2 tiles tall because it looks boring.
LIGHT YOUR MAPS. Use observation rooms and light strips liberally, and feel free to play with alternative light sources if you're a Hammer fan. Light strips are especially fun because you can frame things with them to give little hints 
Get rid of some "extra space". It shouldn't be frustrating to walk from element to element in your test (Chander I'm looking at you
).
Give elements some "breathing room" too. It shouldn't feel tight (RectorRocks plz)
Other things to consider: color, symmetry, asymmetry, structure, appeal, openness, closeness, order, chaos, etc.
(This turned out really rambly but ok)