Here's another one that's hard to figure. The one thing that comes to mind is that there is a variety of "meat" humanoids that are used on a regular basis, be it kobolds, Orcs, Bugbears, Gnolls etc. I think all of us DM's use these types of Monsters to just fill space more often than not, myself included. Maybe we should all start looking outside the box and throw some different monsters in there. For example, rather than throwing in a party of Orcs, why not throw in one tougher monster, a Ogre, a Giant, a Troll (even then they are used a lot). Or for that matter, create your own! Just for something different. One monster that I feel isn't used very often (atleast in experience) is the Rakasta. Create something new and different.