To this discussion I'd like to add some of my own melee map making philosophy.
When you make a map try to balance it out when it comes to open areas versus cliffs. If you put a lot of cliffs then the map can be abused a lot by certain strategies or races, terrans most notably with siege tanks, it can also force excessive air responses. If the map is too open then other strategies can be abused, Zerg become super strong because of the lack of chokes and their potential to swarm their enemies to death.
Balance the map, if you make a certain area too open compensate by making other areas closed. I'm a big fan of having different types of areas on a map so that all races can be equally viable.