WoW is clearly intended to last forever, or at least the game equivalent of more years than can be counted before getting distracted by shiny objects. To do this it needs endless content, something either new or forever (or close enough) repeatable in order to keep people paying.
Obviously content must be created at least as fast as it is consumed. This gives two strategies: create content faster and/or slow the speed of consumption. The former is expensive or may result in a lot of crap. The latter is cost-effective, but can also result in crap.
What's the crap that is risked? Grinds. Make everything take a long time, lots of play time, and you can create many playable hours for comparatively small investment of time. Want to buy more time? Take the current rewards, bump the stats up 5%, and add another level to the grind: more gold, reputation, tokens, whatever. Grinds are efficient: easy to make but taking as much time as the developers want. They're perfect except for being absolutely no fun.
What's the alternative? PvP. Give teenagers fake guns and they will shoot each other all day for years, even without new maps or guns. Look at Halo. PvP can last almost forever since it brings a few tools and then lets natural competitive drives do the rest. The biggest risk is that something better will come along. Of course this example isn't perfect since Halo doesn't have a monthly fee. Still, it shows the possibility of endlessness.
Unfortunately RPGs and PvP don't get along well. RPGs like gear. They like people getting stronger and stronger as time goes on. PvP doesn't like it so much. It works better when new people have a similar footing to those who have been around for a while (excluding personal skill of course). Gear imbalances keep out new people and eventually time will take away the old players, leaving no one at all. But PvP which ignored gear would, well it would do exactly what I said, it would ignore gear. What's the point of gear if not to better crush foes? It could be hard to sell the idea that you only get to be a total badass in the NPC world, but you're as puny as anyone else when you attack another player. It doesn't even make much sense from a RP perspective.
WoW seems to have tried to work around this a bit with arenas and BGs which slowly, but steadily reward losers. This way even a person at the bottom can work up to the top; given enough time they can get the gear they need. But isn't getting slaughtered over and over just a grind, except with other players instead of NPCs? Grinds aren't fun, especially when they involve repeated punches to the ego.
I've not been able to figure out how WAR is tackling this issue. I've had trouble getting interested enough in it. The trailers haven't seemed all that exciting, almost like a bad kung fu flick where it's obvious that no one is actually hitting anyone and you can see the guy jump back into a wall after a light kick. Gameplay didn't seem to catch me either. The graphics were pretty, but somehow the animations seemed lacking. They didn't seem better or worse than WoW, which is exactly the problem, I'd expect more from a game which is much newer and clearly is going for a higher visual standard, at least as far as stress on a graphics is a good measure.