Player-built missions sound like a nice idea in theory. In practice, you'll get lots of Monty Haul
missions and one or two missions that really push the envelope of the mission creator and leave both the players and the devs begging for more.
The Monty Hauls could be weeded out by an Apple App Store style vetting process - the automated tools will be able to check that the rewards match the mission level and intended security level. No officer NPCs in hisec level 1 missions, no 5% implants hidden in level 4 mission structures, no endless stream of under-tanked over-bountied battleships, no shanty town pockets filled with hundreds of structures that have some tiny percentage chance of dropping something really nice. Other processes could monitor the missions once they're available on Singularity and Tranquility, to ensure that bounty payouts and loot drops never exceed certain values.
The two or three really good missions will make it worth wading through all the dross, while simultaneously encouraging the devs to ensure that mission-handling code is thoroughly cleaned up.
There are some nice missions in the new bunch that CCP added to the game - it would be interesting to see what players could do given the same toolset. I'm particularly amused with "pick your poison", since it's a level 4 mission that can be completed (or more to the point is best completed
) in a frigate (similar to Recon (1 of 3)).