One of the things that I like painting the most are faces. I've already done a couple posts about painting human faces (separate ones for male faces or female ones and a third one just about eyes).
A lot of those techniques apply to fantasy and science fiction races also (and many can be applied for striking effect). Often, you have to look at the mini and decide whether using all of those techniques is right for it, or if you should only use some of them (or even none of them).
One race that seems fairly prevalent among miniatures is ogres. And the wonderful thing about ogre minis is that you can usually use every one of your human face techniques on them and the only difference is that you have more area to work with.
As an example, here are some Warhammer 40,000 Ogryn that I painted recently. The faces on these minis were very well sculpted, and I had a great time painting them up.
All of the normal human face details- 5 o'clock shadow and shaved head shadow, red under the eyes etc. -all add quite a bit of depth to these expressive faces.