Lack of an intuitive and really good live help system is my main quirk with all editors, including Emacs. Of course, they all have some included help systems, but they tend to be slow to load (the worst!) or kind of cumbersome. I don't particularly like Emacs help system, for example. It shows you generic documents and it takes too much time to look things up.

If you're writing an editor and want to include a special feature, please consider an online help system that

-- allows you to view help about the mode and keyboard settings immediately by one keypress - with absolutely zero delay, at any time and ideally fullscreen - and return to exactly where you were before you looked up the documentation

-- optionally shows you continuous online help during your editing (like e.g. the contextually most important keybindings while you're editing)