It may come as a shock to some of the more blinkered forum dwellers out there, but different people like to do different things in their MMOs. There are people who live for PvP, which for them means evenly-matched teams in battlegrounds or arenas. Others will tell you that only open world PvP, where there could be a ganker waiting around every corner, counts. Some people just want to get into end-game raiding, some want to enjoy a leisurely and challenging levelling process for each of their many alts, and then there are people who really do want to spend hours messing around with costumes and decorating houses. Each of these groups is looking for a different gameplay experience, and unless you’re making a niche game you have to accommodate several (or even all) of them.
If you have different experiences in your game world, you can either put them in serial (you need to do one before you can do others) or parallel (they’re independent). For example, WoW and most similar games have a solo-based levelling game that you have to play through before you can start raiding. As an example of parallel styles, in WAR you can level up and gear up through either PvE or RvR; a player who wants to do nothing but fight other players can do that from level 1.
As with most things in life, parallel and serial approaches both have their pros and cons. A parallel approach lets each player do what is fun for them, without blocking it behind “un-fun” stuff. On the other hand, it’s more work for the devs who have to make enough game in each style to keep players fully entertained. It’s also very hard to do – even if you think you’ve made a parallel game, if players see one path as an easier route to success on another, they’ll do that – and complain bitterly about AFK leechers in PvP who are only there for welfare epics, or PvPers who feel forced to do raiding to get the gear that makes them “competitive”.
The big mistake to avoid, though, is to design a game that’s got serial gameplay styles and then deliberately favour one over another. This seems to be WoW’s big failing where, as has been pointed out everyone has to play through the levelling game but it’s treated even by the developers as an inconvenience to be got through as quickly and easily as possible. Doing that just pisses off both the guys who LIKE questing and levelling, and those who want to get past it but still have to do the watered-down version instead of being able to bypass it altogether.