My rogue is my Alt, but that may change when he gets to 70 (at least that's the way I feel about it now ... yup, cos I think he owns big time ;->)
My first toon was a warrior (level 70 fury now), and I guess he's my main .. I've spent the most amount of time building him, and he is clearly my most kickass toon.
My first alt (and one that I do instances with mostly) is my 70 Mage ...
I wanted a toon as a first alt that was different in every way to the warrior, and I figured that the mage was that class.
The rogue I rolled cos of the fact that I have the warrior. We all know that Fury Warriors are wannabe rogues - it's true I tell you ... very very true.