Never fear, we've got social features for days.
In order to prevent fully impossible levels from being uploaded, creators in Super Mario Maker have to prove they can beat the level first before uploading it to the network for other players to attempt. This information comes from a recent Treehouse Live demo Nintendo held at E3, where they revealed additional details about the upcoming game's online functionality and sharing features.
It appears that based on how many people complete the level, the difficulty can be dynamically adjusted to be easier or harder as well. Level creators will also get play counts for their levels at any time. And when searching for levels, you can narrow the search down by difficulty, time uploaded or location, as well as grabbing Nintendo-featured levels or newer, "up and coming" levels. Players will also see the clear rate, grab a pictorial overview of the course, and leave comments + see where people die during the levels. Comments can be toggled off or just left on the TV if you're playing with off-TV play on the Gamepad.
Should you like the work of a particular creator, you'll be able to follow them in-game and automatically see when they upload new levels.
Screen caps courtesy ZSaberLink from NeoGAF