I don't have a problem transitioning between characters. Using the same key binds won't be a problem. After hours of playing and adjusting to the keybinds for the first time, muscle memory will take over. Changing up what abilities are bound to what keys, however, will throw you off for a bit.
There is math behind why mouse clicking is slower. It's called Fitt's Law: http://en.wikipedia.org/wiki/Fitts's_law. It really only makes a difference if you can't make it in time before the global cooldown. Not all abilities are on the GCD though. In that situation, the dps is negatively effected, but not by much. Over the course of a boss fight it adds up. Even if you can make it before the GCD, you're taking your eyes off the situation going on around you, which is much more important. That time spent searching, moving your mouse over, and clicking could be better used watching for aoe/interrupts/energy management, just a plethra of things that would be more beneficial. Also if you're a mouse clicker, you probably aren't turning with your mouse. Turning with a/d or arrows is just so inefficient. Strafing and mouse turning are just so superior in any situation. Backpedaling is another no no.
