Many safeguards are necessary to maintain the playability of the game while being made of user-generated content.
In order to have an area uploaded to the servers, the player that creates it must first complete the area without taking a single hit. This should not be a very difficult accomplishment, because the creator knows the position of each danger, and thus how to respond properly.
Fundamentally, the gameplay is designed to empower the player. The controls are tight and instantaneous (You can turn around on a dime, completely change the direction you're moving at any time, even while jumping), and enemies follow the same rules as the player, being just as vulnerable to traps and the attacks of other enemies. Each trap and enemy is simple to overcome by itself (skilled players can even swat down fireballs with their sword), and difficult level design comes through clever combinations.
As well, I am working on an algorithm to detect areas where players can get permanently stuck (Such as a big pit with nothing at the bottom). On top of that, I'm working on implementing a feature to let the community blacklist areas that are deemed unfair (such as if it contains an unavoidable element of luck, like if there were two pits that were too deep to see the bottom, one of which lead to the exit and the other leading to certain death).
Branching paths are being considered so that players can choose whether or not to venture into an area that's been proven to be difficult. The success/failure ratio of each area will determine the amount of treasure that is found at the end, giving an incentive to push yourself, but no real punishment for taking an easier route.
The ideal level is one that is difficult, but manageable with the proper strategy. Levels where players give up and return to town are less likely to get chosen by the server than levels where players die a few times but ultimately finish.