I think a lot of old school WoW players will agree with me that the Scourge Invasion event that kicked off the Naxxramas patch (and coincided with that year's Midsummer Fire Festival) was a whole lot of fun. There was something incredibly epic about fighting a massive hulking abomination on the steps of the Stormwind Cathedral that not even most boss fights have been able to capture for me. it was also nice to have all those extra bosses in the dungeons for a bit of variety, especially since some of them dropped some very nice loot. Beyond all that, it finally gave us a chance at getting the much coveted tabard of the greatest faction in the game.
Still, despite Blizzard's hints that we'd get a chance to experience the Scourge Invasion again, it's been over a year and a half since the citadels ceased to hang over our capital cities with no sign of a return -- I've even saved a space in my bank for my spare Necrotic Runes, just in case!
Of course, if the Scourge Invasion is ever to return, it's getting closer and closer to the most opportune moment for such a thing. WotLK is coming, in which we will invade Northrend ourselves to try to put a stop to the Scourge once and for all. Preparations are already underway in Stormwind for the journey, and rumor has it that Arthas himself is in eager to lure us to Northrend and trap us there. What better way to do that than with a preliminary invasion as bait?