Check the skills lists and see :)
I have only been defeated by a paladin in a serious duel. Oh and I never had the pleasure of being challenged with a shaman :P . However, rogues can be rough with that stun of theirs :) Thank goodness for frost nova.
So yeah, a ton of people are frustrated with the fact that they might have to think once in a while when playing the mage class. So, they pick some other class, then they duel a mage, and get turned into a sheep, come onto the forums and complain about how stupid polymorph is. So, naturally, the majority of the conversations you hear about mages involve the word "polymorph" or "sheep." Despite what the context of the conversation is, after a while, you just start seeing those words associated with the mage class.
Don't for a second believe that Mages are limited to a sheep spell :)
Oh and one last thing, don't go shaman unless you would really enjoy that type of class. Your reason shouldn't be because everyone on the forums talks about how awesome they are.