what I think is happening is that the class designer for mage has been given some design directions but he/she did not really run with the ball too much. It could be either the direction or vision was not clear or did not tie in well enough with certain things. The warlock designer seemed to have gone about it better in the eyes of many.... the close attention of a certain person might have been really helpful.
its kinda hard to make the mage any better without overpowering the class in pvp. Fire was next to useless in arena while being immensely useful for pve. Arcane is marginal while frost was exceptional. It does amuse me to think they tried to reign the class back in at the beginning only to realise that the other abilities they gave to the other classes worked out better than they thought it would. Nothing changes the fact that people still complain of Mages being OP while the mage community largely feels that the mage is in line. Its evident in their zeal to bring warlocks into pve, they buffed them a little too much.
2nd expac, a more diverse raid environment that has been around for a year and a bit. Managing class balances might be a little bit easier this time round.