Idea behind the mage is obviously a limited amount of damage one can do at an accelerated rate compaired to other classes.
Problem with this is, though, that this really doesn't happen till quite a bit later in the game - meaning a Mage can do about the same damage as a Melee but with a limit of mana. It does sort of suck. That aside, though, you'll see ALOT more control with a Mage than you had even with your Rogue. A lot of CC options, ways to handle yourself defensively and offensively, and with a good spec you'll find you can push out some Rogue-ish damage in a pinch.
Put all of this with the fact that you can start fights off from a decent distance and you'll find yourself grinding Yellows/Oranges who never touch you...
I think saying it's "hard" to level a Mage is false - just maybe takes some sitting and drinking every couple of monsters.