YouTube Collections are grouped subscriptions, basically playlists of particular channels or feeds, that can be labeled with a single title and viewed in a new playlist. They're handy, but apparently not enough people were making use of them. According to service messages on both the Collections pages on YouTube itself, and on the YouTube help page for the Collections feature, they'll be going away soon.

As usual, Google and YouTube gave no explanation of the change aside from "focusing on other efforts." Considering how well the standard YouTube subscription feature works, and how deeply integrated it is with the rest of the service, I can't say I'm going to miss this extra layer of customization. Of course, others may not agree. Feel free to express your disinterest and/or blinding rage in the comments sections.

Note: no relation to Google+ Collections.