For example, if you pause the video by clicking the main action button brings up an overlay that takes up almost the whole screen, so you can no longer see the content in case you paused to freeze the frame. How do you start it again? By clicking the same button, right? No! By clicking up. For some reason up means back and down means to open some additional UI with related videos and what not.
No other app is like this — Plex, Infuse, Apple, Netflix etc. abide by relatively sane UI controls where the action button pauses and unpauses, and up/down don't scroll between weird overlay elements.
The YouTube filled with these incredible non-unintuitive UX choices that drive me crazy. I never use it unless I have a clear idea of something I want to watch.