YouTube has a new interface for related searches and advanced search options. Now you can select multiple filters from a category and combine related searches. For example, you can find long-form HD videos that are available in the WebM format.
If you click on one of the queries from the "Explore" section, you'll replace the original query. A more interesting option is to mouse over a query and click the "plus" icon: this way, you'll refine your original search.
YouTube adds both filters and refinements to the query and separates them by comma. A search for [slow motion, hd, webm, high speed camera] includes two filters that are treated separately: "hd" and "webm".
You can also add the name of a channel (example: [slow motion, @tpsfilmstudio]) or the name of a category (example: [slow motion, sports]).
The new features were part of an experiment called YouTube Topics that tried to help users find videos. "We know that sometimes people come to YouTube looking for a specific video, but at other times, they have only a rough idea of the kind of videos they want."