It depends on the map..
You must place objects in a logical way and make sure that the objects don't repeat too much or get placed in the wrong places.
e.g. How would you like cs_estate with a whole loada crates in the offices? That would surely make the map less fun and unlogical.
And yea, Kasperg is right... U need multiple paths to the map goals. But dont start making the map too complex. 3 or 4 different aproches to the goal are farely enough in a map.
If you're mapping a de map try making the bomb harder to defuse. Like in de_dust or dust2.
Also you must balance the map so that both teams would have an equal advantage to their goal. Like in de_deep6 the T's mosly win if they plant the bomb because of the small number of aproaches to the bomb site.
Use ur imagination. Make ur map stand out of the crowd. Not much ppl like those de_dustathon555k maps... (at least i don't). Try finding or making your own textures to make the maps' enviroments just the way you want them to be. Guess thats it...
Geez i've spent 5mins writing this!!!