I know from "What is this new icon in some of the skills?" that the water droplet icon means that a skill can't be used underwater, and the cooldown icon is easy enough to figure out, but what does this clock icon mean?
This was recently added in the October 7th patch, though oddly enough they don't actually list it there. That symbol represents the activation time and/or channeling time (if it's a skill that channels) in seconds for a skill. Anything that provides Quickness will reduce this value (by half).