WoW's age is no excuse for tired, rehashed gameplay. I still have fun with the game, I still subscribe, but I no longer do a lot of the things I once wasted hours obsessing over. I'm not doing daily quests that never change, never seem to truly effect the world, etc. WoW gets by with this because there is no competition. I hate to sound like Etoilet, but GW2 changes that.
The raids are still fun, but even those feel like rehashes at times. But the problem has almost always been that once you get to the end game, all you can do is raid. There's not much else to do when you think about it. So basically with each expansion, Blizz released a raid that guilds obsess over for months, then once its beaten they spend even more time doing it over and over again. So there's constantly this huge group of end game players who wait for expansions or patches just to get the next raid to do a billion times.
I'd love to see more world events for end game players, raids that feature some random new challenges/bosses each week, etc. Something, anything to keep things at least semi-fresh.