Arenas are still the best environment to highlight PvP skill but have awful replayability. Nobody plays unrated arena matches for fun. Teams that play unrated matches do so in preparation for rated games. In between arena seasons, the arenas are a ghost town and battleground queues are healthy. Never mind that Blizzard disables arenas between seasons; nobody would play them, anyway. A good many teams play the minimum matches required to qualify for arena points and ratings every week, because unless you're a hardcore PvP player, arenas are a chore. The biggest reason anybody even plays them is because item rewards require ratings. Arenas are only intrinsically fun for death match aficionados, whom I suspect aren't a huge population in a fantasy MMO. The format lends itself well to pro gaming, which is the best thing about it. But as something players can repeatedly play, it fares rather poorly.
Quests are fun, but save for dailies, you only go through them once. Dungeons are a little better, and many see repeated visits. Unfortunately, dungeons are also tied to level, which means characters outgrow them eventually. This is why heroic Deadmines and Shadowfang Keep will be so exciting -- they were fun while they lasted, but after a certain point, they're no longer challenging or worth the effort. We'll enjoy both retooled dungeons for a good while at level 85, but when the next expansion rolls around (maybe one that pushes our characters' max level to 90), they'll lose their relevance again.
Thanks to the emblem system, dungeons retain their relevance in the end game, even if players no longer need or desire any loot from dungeons a little below their level. This has allowed most end-game dungeons or heroics to retain their replayability, becoming farming grounds for currency to purchase better gear. In a way, it was already reflective of how the battlegrounds worked. Players farmed honor to purchase gear, but it was always a limited selection. Still, this did nothing to diminish battleground participation. If anything, for some players, gear was a secondary concern in battleground play. The fun is in all the fighting.
Fun and stress-relieving
One of the greatest things about the battlegrounds, in fact, is that when everything else about the game gets dull and boring, players can always play a few matches just to spice things up. It's a fun activity, especially when done with a few friends. Even if you're not big on PvP, there's no pressure in battleground play as opposed to arenas, where the rating system allows very little room for error. Rated battlegrounds in the Cataclysm will be a different story, but even then, accountability won't be as heavy as in arena play, where one missed heal or uncoordinated switch can mean eventual defeat. A battleground loss is difficult, if not downright unreasonable, to pin on just one person. This removes a lot of the stress factor, which makes it a great way to unwind.
The continual resurrection is one thing it shares with first person shooters, where players die and respawn almost immediately, allowing them to get right back into the fray. Arenas or even dungeons aren't like that. Deaths hurt. The battlegrounds allow players to actually use death strategically -- I mean, fire mages can do suicide runs in the battlegrounds. Can you imagine that in an arena match? Or even a dungeon? It's a different level of pure, unadulterated fun.
That's the thing about the game. We play it because it's fun. The game stops being fun when there's nothing left to explore, to gain or to do. As Wrath of the Lich King's chapter closes, the number of things to do starts to dwindle. We've explored all the dungeons, beaten all the bosses and finished all the quests. The game slows down now and we need a little something to keep us entertained until we can reroll our goblins and worgen. For me, the battlegrounds have always been there to bridge the gap. In fact, when Blizzard deploys new talents and abilities as a patch prior to the launch of the next expansion, just as they did with Burning Crusade and Wrath of the Lich King through patches 2.0 and 3.0 respectively, the battlegrounds will be one of the best places to try them out.
Always in moderation
Needless to say, there's always too much of a good thing. Playing too much battlegrounds can be nauseating, for sure -- imagine the old-school grind where honor (before it was a currency) decayed and your gains were measured against other those of other players. This meant that in order to move forward, you needed to PvP more each succeeding week, to the point where playing the battlegrounds in excess of 20 hours a day was mandatory to move up in rank. The game has grown up quite a lot since then. It says much about the battlegrounds that they are still relevant and fun to this day, even with all the content already out there.
This is why Blizzard's decision to create a lot more battlegrounds -- I wouldn't be surprised to see the number of battlegrounds double at the end of Cataclysm's life cycle -- is an immensely prudent one. There is very little content in World of Warcraft that is as enduring and replayable as the battlegrounds. It pushes the story in very little ways, but is always thematically on point. It never grows old, even if it actually is. Warsong Gulch can still deliver as much fun now -- perhaps even more after it was tweaked -- as it did when it was launched in patch 1.5 nearly five years ago. When Twin Peaks and the Battle for Gilneas come out in Cataclysm, we're pretty much guaranteed to be playing them throughout the entire expansion and beyond. When Cataclysm winds down in a couple of years and players are hit by the same lethargy and near boredom that usually comes in between expansions, the battlegrounds will continue to be there to help get us through the day. Pantawid gutom, you could say. Something to munch on before the feast.