I can think of two extremely annoying factions: Alliance and Horde. Seriously
- Constantly at war with one another
- Accuse one-another of the exact same thing, all the time
- Look to be different in culture, but the conversations on trade chat shows they share the exact same jokes
- Whine about the same nerfs
- Both threaten to cancel "their subscription", whatever that is in Azeroth
And that's just a sample of why both are annoying.