My opinion?
Too much multiclassing allowed. Multiclassing should have been restricted to one or two "similar" classes and not allowing each and every class on one toon.
Ny reasons? Multiclassing one toon with 5 classes is a lot easier then bringing 5 toons up to 100 once. So, once everyone gets multiclassed out the whazoo the game quickly gets stale and boring with nothing left to do but mass kill and farm. Not many seem capable of playing a pure mage or a pure fighter or anything pure.
To make the game soloable they allowed too many skills to be crossed over, which once multiclassed 10 plus times allowed the mass killing. To compensate dragons for being a single class and behind a multi biped they powered them up as Ancients.
So to finalize, the game balance is totally out of whack. As far as I am concerned at least 1/3 of the MoBs in the game SHOULD NOT be soloable by anyone,no matter how much power they have or how many classes they obtain.
And to those that say multiclassing is what makes the game unique I disagree. Rampant multiclassing is just a crutch for lack of game content, nothing more.
Edit: Restrict multiclassing to one or two similar classes eg fighter/healer/mage, add an extra 5 character slots so everyone can make up different toons and not just rubber stamped 10+ classes of the same toon.