I personally think User Created Content would be HUGE for WoW so many people get tired of doing the same old thing in regards to questing and dungeons i think the addition of UGC would breathe a lot of fresh new life into WoW.

Now there of course are potential issues that could arise such as abuse of the system and monitoring of it. For example creating really great gear for really easy dungeons or tons of experience or stuff like that. But i think this could be avoided given given some rule systems that were put in place and good moderation.

The upsides though i think out weigh the negatives.

Gear and other rewards could be defined by the difficulty of the dungeon. The system would measure how many mobs and what type of mobs to determine the rewards (valor, justice and or gold) as appropriate. Then you could pick from a list of pre-made bosses to add and they would have their own drops determined by their difficulty.

This would allow the designer to determine how hard they want to make their content and players would be able to choose if they are willing to do harder content for better gear and rewards.

Adding to my first comment, they could make the type rated content into heroics. Gives designers motivation to make really good content and hope to be chosen. Maybe give designer rewards like gold for everyone that runs their content, or some point system like justice to buy gear only available to content creators.

There is so much WoW could do with this type of system that I hope they take a serious look at it.