Fine until you raid with a mage *and* an ele shammy, as I used to :-). There aren't enough marks to go round, and different folks use them in different ways. Blue for freezing trap seems to be remarkably common, though.

The other thing we tend to do if operating with very few healers is to mark the tanks. Occasionally we'll use a mark for a "key player" - in the early days of doing OS, for example, we'd mark the caster who was best at dodging fire walls and tell the other casters to Watch Him And Stay Near, Dammit!

These pretty much line up with what I have always used. Moon has always been poly or something magical and Blue Square has been ice trap for obvious reasons. Although circle tended to get a fair amount of usage as sap and purple diamond was usually associated with some warlock ability like fear or banish. Now since no one hardly uses any CC it doesn't matter much anyway.

@Ozzard:

Yes, the ones that I've seen be the most consistent are Square for the Freezing Trap and Moon for Polymorph. (There was even a guild on an old server of mine called Sheep the Moon.)