Mojang is updating their commercial use guidelines!
Wait, don't worry just yet; these changes mostly affect advertising agencies and corporations relating to promotions, and has virtually nothing to do with end-user monetization (such as one uses for Youtube, servers, et. al).
Mojang's Owen Hill gives a breakdown on what this means:
If you are an ad agency, corporation, non-profit, or politician, you can’t do these kinds of things or hire someone to do them:
- Build a Minecraft mod or server that promotes unrelated products in playable form; e.g., if you are a restaurant chain, you can’t market your restaurant by releasing a mod that includes your restaurant built out of Minecraft blocks.
- Build a Minecraft map or mod that markets a movie or TV show; e.g., if you’re a movie studio, you can’t make a map that uses Minecraft blocks to build out the fictional world of the movie or its characters, and you can’t make an official movie trailer out of gameplay footage from that map or mod.
- It’s worth mentioning that fans of a particular restaurant, movie, or some other thing are still free to build things in Minecraft that represent or celebrate it so long as the goal or focus is not to promote or sell that stuff. The new rules only apply to companies or organisations that are using Minecraft to sell their products or promote their causes.
In short - companies that want to use the game as an advertising engine by building their fictional worlds, business locations, or similar for the sole purpose of advertising their product are not allowed to do that. However, players are still both allowed and encouraged to build whatever they want. That's a fairly broad simplification, but you get the idea, hopefully!
If you (the reader) are not a corporation or advertising agency, there is essentially nothing to worry about. Have a great day, everyone!