Maybe it's because my first MMO was Neverwinter Nights, a 1991 game that took the Gold Box DnD experience online, but I think WoW has only gotten better with every expansion. When it came out, most people I knew disliked it for being too simple but agreed that it had better atmosphere and polish than any other MMO. It felt like MMO Disneyland. The initial mechanics for raids and PvP were limited, but as the designers gained experience, they kept coming up with new and interesting mechanics, and the art and atmosphere maintained their high standards throughout the years. I haven't had time in the last few years to spend on high-end raiding, but I've heard nothing but praise for the raids from the top players.
The only place I see it failing is figuring out a way to keep communities together over the years. Most people's main experience with the game is social, and if your group can't stay cohesive, and few can for 12 years, you're going to get a diminished experience. It's simply an impossible task to figure out a way to keep gamers together for that long.
Other people will complain about parts of the game being too easy, but I always dismiss those out of hand. The high end parts are more challenging than ever, and the existence of diverse content is nothing against the game design. I love that I can log on for a few minutes here and there and even only play Pokemon if that's what I want in the moment. As an old-school MMO person who never saw WoW through the eyes of novelty, my perspective is that no game has managed to maintain such high quality and consistency while continually reinventing itself over such a long time. The fact that it's still the number one MMO by far and maintains a subscription model while everyone else is going free-to-play is testament to its quality. I think it's a piece of software that any developer should look at for inspiration on how to keep people engaged for a very long time.