Not having a class system would make it feel weird joining a class-based faction, like a mages' guild or Skyrim equivalent. How can I join a (class name) guild if there are no (class name)s? Unless they do away with factions with class names and just have the College of Whisperers (I think that's it?) and some other Nord-based one, like how the dunmer had the Great Houses.
Quite simple, in the previous TES games there are skill requirements to join the factions and to gain ranks within the faction. For example, Morrowind had http://www.uesp.net/wiki/Morrowind:Mages_Guild#Mages_Guild_Ranks.