For me, it's definitely the un-skippable advertisements. Like, as a software developer and user, I despise when an API is exposed that allows an application or website to suspend expected behavior in favor of custom hijinks, but to me, that's exactly what ignoring my menu and main-menu buttons is.
Especially when it is enforced by licensing terms. At some level, it is just a bit that gets interpreted by the software as "ignore these user commands". It is very reasonable to make a player that listens to the user commands, even if that bit is set. At a different level, it is a requirement under the terms of the patent licensing to implement this "feature", or else be in violation of patent infringement.