Sure, getting together with 40 of your best friends and leading an epic raid on the alternate faction's capitol city can be fun, but I have to say that there's fun to be had doing things all by yourself, too. Some classes were soloing Onyxia at level 70 -- and it seems like just about everyone can solo her at or near level 80. (For those thinking of trying this yourself -- it's good fun and not a bad amount of gold for the effort.) Zul'Gurub and Ahn'Qiraj (the 20-man wing, at least) have both been cleared (mostly) by a lone player. Karazhan has been cleared with a group of 3 -- though in our post-Wrath world, I'd imagine that everything save Netherspite could be easily managed with less. (And at some point even Netherspite is bound to be overcome by brute force.)
To solo or not to solo, that is the question. Whether 'tis nobler in the mind to suffer the slings and arrows of outrageous Hunters (who sometimes seem to think they have a monopoly on soloing content) or to take arms against a sea of dragons (dragons, elementals, demons -- we're not picky), and by opposing, earn epic (albeit under-level) loot. So, tell us, what have you soloed lately?