Put stuff in. Don't make the level completely empty and devoid of stuff. After all, stuff is good.

If it's a single player level, only put one or two (max. three) weapon powerups in. If it's a battle/treasure hunt level, put in as many as you bloody well want. People will enjoy flying around and shooting stuff, and doing twice the damage. If it's a race level, put in mainly ice and RF ammo, because nothing else will really work well on the run. Lastly, if it's a CTF/assault level, don't put in too many powerups, or else everyone will die too fast. Remember, in CTF, you only have 3× health, instead of 5×.

Oh, one other thing: I got this advice from a pro levelmaker, although I can't remember which one it was. Split the level up into sections. Create one section at a time. Decide what you want in each section. Completely finish that one section before you move on to another one. Then, when all three, or four, or seventy, sections are finished, connect them all with a few unmasked pathways between them, and you have a HOOJ, insteresting-looking level.

IMO.