Valve in particular saw a lot of success from their efforts to promote user-made maps for “Team Fortress 2”, later expanding this to other types of content and other games as well. Valve’s fans were quite happy with the situation, but without doubt the ones who were truly happy were the content creators themselves.
Because back then, Valve also figured out a way to let those people monetize their work, and there were reports of people receiving quite the large sums from the company for their contributions.
This trend is still alive today in “Dota 2”, and many players are actively contributing to the content library of the game, regularly releasing skins and other kinds of modifications which then end up earning them good money.
We don’t know if 343 are planning a more advanced system like that for their own game, and for the most part people are already well motivated to contribute to “Halo 5”. The negative feedback about some maps seems to have actually inspired their creators to do better, and now we are seeing a surge of interest in this kind of work. We actually wouldn’t be surprised if even more maps than before started to come out on the market in a few weeks, and they will likely be of better quality as well.