Rare mobs are one of my favorite unexpected pleasures in WoW. It's such a thrill to be questing or grinding along and see that silver dragon; it adds a lot of flavor to what could otherwise be some boring runs through out-of-the-way zones. In fact, that's an upside to the current depopulation of Azeroth: I find many more rare mobs, since no-one's been by to kill them in an hour or two. Fun fight, interesting mob, automatic green.
However, when Burning Crusade came out, it was discovered that all the Outland rare mobs were also elite. There was a blue post around the time that defended the decision as allowing them to put better loot on the mobs, making them walking treasure chests (
BC also has no world treasure chests, sadly Thanks; I guess there are still new things for me to learn in BC) . But it did make them basically unsoloable, which takes a lot of the excitement out of spotting one, at least for me: by the time I get four more people to come help me out, I don't really care any more.