To be honest events that pops every year are boring, because when you did brewfest 4 (5?) years in a row it's always the same. first brewfest was great, i managed to get the non-epic ram mount though

and they managed to remove the quest with pink elephant in EU, because it would change the PEGI of wow (if i'm not wrong). That quest was hillarious.
Look at dota2, their events are always different (yes i know it's a lot of work), blizzard is getting really greedy on this.
Usually events before the release of a new expension are great. I liked that scourge invasion when you could play as a zombie yes! The elemental invasion pre-cata was ok. The event pre-mist was terrible, doing it instanced is the worst idea ever.
I also liked the event where you had to rescue the gnome king and get that gnome transformation as item, because it's something unique that won't happen again (this thingie should be a feat of strengh btw).