Some things that spring to mind:
Don't make room after room of randomly assorted of monsters. Have areas with one monster type, or a few, and a few others with a big mix.
Introduce new monsters with purpose, as a means of presenting progression in a map.
Mix up monsters on the same floor of the player and projectile monsters on ledges.
Don't do cheap behind-the-back monster closets/teleport traps. This makes the player creep boringly throughout the map.
Don't spring monster closets/teleport traps every time the play picks up a new weapon/key/powerup. This is bland and predictable.
For high-hitpoint monsters try really hard to not create cheap spots the player can abuse to take the monster out with no fear of damage. This includes corners and ledges. Monsters like mancubi, arachnotrons, archviles, and spider masterminds are some of the tougher monsters to place well because of how easily a player can abuse corners with them.
If a player can run circles around the monsters and have them all kill each other, chances are it's bad monster placement/map design.
Try to make monsters activate before the player comes upon them. Always knowing any monster that will be encountered is going to make a loud roar and announce its presence reduces tension. Active monsters also tend to attack faster that those that activate upon sight of the player.
Make sure your monsters can navigate stairs (and any other map geometry) unless you desire otherwise.
Reduce the number of doors in your map, or place a few weaker enemies on the other side of a door. Door combat is garbage.
Don't have a spot where the player has to repeatedly use a lift and snipe a few enemies at a time who have shuffled on to it.
Use damaging floors, crushers, or dark areas to limit the player's movement while providing more area of movement for monsters.
All these, of course, shouldn't be followed to a T. A good map designer could ignore these rules and still make a great map. These are the ideas I tend to keep in my mind when I'm mapping, however.
Oh, most importantly:
Never, ever, ever place revenants in your map.