It's simple.  The best alliance race for a mage is a gnome hands down.  Solely because of the 5% intellect.  Let's say you have 500 intellect, not TOO terribly difficult to obtain.  With 5% intellect that's an extra 25 intellect.  That equals .3125% chance to crit and 375 mana.  If you are arcane, with Arcane Mind, which gives 15% intellect, that's an extra 4 intellect, brining the total up to 29 bonus intellect which is .3625% crit, and 435 mana.  Throw in Mind Mastery which gives 25% of intellect to spell damage, that's about 7.25 bonus spell damage. 

Wow, now imagine somebody coming up to you and saying, here, have a free extra equipment piece that gives 25 or 29 intellect, are you gonna take it? 

((never played horde so I'm not one to judge))