I don't play World of Warcraft, for many reasons. Almost all of them are personal preference; as much as I don't like WoW, I can't say that it's bad. Far too often, people mistake their own opinion as fact, and I want to go ahead and put that out there before I go on.
Now, there is one thing that Blizzard has done, that I hate, and it's the repeated increase of the level cap in WoW. Also, by proxy, how this practice has lead to droves of people carrying over a faulty logic to other MMOs that, in increasing the level cap, one can make a game more fun.
Firstly, why I dislike increasing the level cap in an established, persistent online game. It tends to invalidate the work and effort of a player. If I've spent the last two months raiding with my guild, to complete my Tier # armor, I'd feel pretty shitty when the developer of the game decides that everything my character owns (and that I thought was good) is now worthless junk, because there are ten more levels for me to grind out, and each one has gear more powerful than the last.
Now, increasing the level cap certainly isn't bad for newer players, or ones who haven't dedicated depressingly large amounts of time to pimping out a character. It sucks for the hardcore players. Of course, they are also hardcore enough to suck it up and grind out those ten new levels, and work up to the newest Tier of gear. I sit somewhere in between casual and hardcore, where I care enough and play just enough to be well equipped, but I haven't religiously cleared the same instance dozens of times to earn a particular set of gear (this doesn't exactly translate into CoH and GW, but close enough).
This brings me to the crux of my rant; I'm tired of hearing people suggest upping the level cap as the best way to make a game better. Wrong, bad, fail. Firstly, not all games revolve around high level play. Secondly, not all games have an endgame the involves endless grinding for loot. For example, City of Heroes is more about the (admittedly downplayed) story of a world recovering from a massive invasion, and your role as a fledgling hero trying to save it. You start as a scrub fighting riots and taking down low level drug dealers, to traveling between dimensions, defeating evil super-powered despots and liberating entire worlds from tyranny. The scope of such an evolution is hard to appreciate if you blast past low level play to get to high levels, and it's completely lost when you replace one plateau of power with another one.
Guild Wars is another great example. While Prophecies, Factions, and Nightfall vary greatly in their PvE application, it's undeniable that the endgame in Guild Wars is PvP. Lots of PvP. Coordinated, uncoordinated, pro, scrub, hardcore, softcore PvP. Wanna do small scale? Arena. Large scale? Alliance Battle. Ultra cut-throat internationally competitive? Guild vs Guild or Hall of Heroes. Increasing the level cap offers absolutely no benefit here.
What can make a game better? What can extend it's life? Additional content, not at the expense of the effort players have already invested in the game. City of Heroes publishes a new issue of the game every three to four months, adding more content each time (including but not limited to: New Powers, New ATs, Inventions, New Zones, Power Customization, New Villain Groups, etc). Guild Wars released an expansion meant almost exclusively for max level players. Additionally, they added (early on) the ability to play all the low level areas as elite lvl 20+ zones.