I usually think of mages as the jack-of-all-trades variety. All they've got going for them is their spells, but in the words of a real-life wizard, "there's an app spell for that". Whereas the rogue/thief type are specialists in the variety of skills needed for infiltrations, I think of mages as more general than that...most of the time. It's always possible for them to specialize themselves into one of the wide variety of things that magic can do, such as Blowing Stuff Up, Summoning Minions, etc.

Basically, while the Gandalf mage Does Everything Well and the Archer Mage Does One Thing Well, for protagonists/player characters I like mages who Do Everything - including Surprising Things - Averagely through clever application of their spells.