Super Mario Maker will allows players to create and upload levels to share with the rest of the community. However, as with all user-generated content, the quality and level of difficulty will vary greatly.
The only rule for Super Mario Maker levels that are shared with others is that that the level needs to be completed, e.g. before submitting your own level, you actually have to play and beat it. Only then can it be available for other players to download.
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On top of that, Nintendo moderators will be manually checking the levels for difficulty before the level is allowed for download. There will also be a somewhat automated difficulty setting: if only a few people who download a level manage to complete it, it will be labeled as a more difficult level, naturally.
Recently, Nintendo and Facebook teamed up for a Hackaton where Facebook employees created their own Super Mario Maker levels. You can check out a video of the winning level here.
