Skull: Kill this
Everything else: Consecrate. (Sorry, I had to.

)
OK, now for a more honest one:
Skill: 1st kill
X : second kill (seems to be relatively universal)
Square: ice trap
moon: CC (usu sap or sheep)
triangle: another CC (rare that Iget this far)
I like square for hunter trap, as I always preferred that on my hunter.
I use moon for something to be CCed -- sap, sheep, whatever. If I have multiple such CCers, I ask them if they have a preference. I've had mages ask for the condom, or for the diamond, for example.
Normally, I'll say, "... okay, I'm using the moon for the {sheep/sap} target.", and that's fine. When I'd mark as a hunter, usually we never got past 2ish CCs in 5-mans anyway, and as a paladin ... Just Kill the Skull and then assist me.
Then again, I'm a lazy raid marker.