My understanding of the classes is that they are designed with specific roles in mind (IE: Rogues and Rangers are the Heavy Hitters, Mage and Druid are for AoE.) Sure you can make them something else but they will never reach their full potential as a class if you do. I prefer my mages to be "glass cannons" that have high damages vs. both single and multi-targets but cant stand up to attack very well. This is most likely a case of me being stuck in the past. I think it may have been interesting to offer each class a offensive, or defensive, or hybrid build.
EDIT: Im also slightly concerned that since spells don't scale with level that if you miss finding whatever the next spell upgrade is that the mage will fall behind but hopefully the new spells aren't too hidden.