Hi Bill and Dan, I've been on a long summer break but I hope to be back more frequently. I have a few ideas:
1.In my own drumming growth, I've found that forming an analogy between drumming and language helps me understand both drumming and music on the whole a lot more. For example, there are times to speak loudly or softly, times to shout, times to shut up quickly, and sometimes when in a conversation you find yourself finishing the other's sentence. These elements could all be applied to drumming. Although I'm afraid I can't describe in too many musical terms what I'm experiencing, I'm hoping you guys' music experience could help me put into words what I'm seeing.
2.I realize the extra things you do while you're drumming could really affect your expressiveness on the drums. Sometimes I hum while I'm playing, but get this, my humming has nothing to do with the song (?!?!). Somehow it helps me get through fills. Also I find playing in an animated and expressive way helps me to play more confidently and more creatively. (Eg Billy Kilson) (One of my new favorite drummers).
3.I mentioned above forming the analogy between drumming and language. Well I also sometimes form the analogy between drumming and colors (like cw said above). For example, the higher toms remind me of brighter colors, or happy times, and the lower ones remind me of sad or sobering issues. So I believe forming fills around this principle helps me to form suitable and unique fills for songs.

So these are my ideas based on my understanding so far. Hope they help!