I should preface this with a note that I played Afflock back in Vanilla and raided in BC and WotLK. I only really switched to my Mage in Cata because my friend is playing on the Ally side, so I've actually had more raiding experience on my lock than my Mage.
Afflocks in BC days were also also one of the top DPS specs, but the rotation was punishing. You couldn't clip your DoTs, no AoEs and you have little to no movement abilities. But if executed well, you'd top the DPS charts although people complained ceaselessly about how tough it was.
But today, you can lay all your Dots with 2 buttons, DoTs can be refreshed anytime after the mid point, you can move and cast/channel without much repercussion and they have battle res. And yet they can still top DPS on single or multi-target fights (take that SPriests!). They are all-rounded in just about ever aspect.
In comparison, mages may be competitive in DPS, but only if it is not very mobile. And personally, I have found Frost even more challenging to play than Affliction right now (timing Evocation is not trivial!). In terms of utility, mages were almost vital to the group in the past because of sheep, but now, even our cakes are discarded in favor of feasts.
Sorry for the rant, but I just can't get over how much more developed as a class Afflock is now. I'm sticking to mage now, cos my team already has a lock, but I really hope that devs can also develop mages to be a more well rounded class.