A lot of people say the hunter's double jump is the best class moving ability. While I really like levitating as a warlock, I think that ability would be a hell of a lot more useful if we could aim and fire our weapons while levitating. As it is now if you levitate and ADS or fire, the levitation cancels and you fall to the ground.
I can see a lot of tactical potential in being able to shoot while flying. If you're floating sideways you could fire down at any hostiles on the ground while making yourself more difficult to hit. It could make levitation an extremely deadly ability if used well.
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The second underclass of the warlock has a skill that hold you a short time in the air when you aim ;)