Memory usage, extreme lag, the bugs (apparently I need firefox 37+ to access the voicechat feature, I am at 65 though), centralized (a big issue considering the censorship that they apply), no way to use it over tor (as far as I know at least), you are not allowed to make/use a 3rd party client, the official client is closed source, no e2e encryption or authentication, etc.
Discord wastes a few seconds of my time every time my Mac boots up because it can't just open at login without stealing the focus once or twice for no good reason.