I believe that doing a mix of the three methods of leveling would be the most effective; those methods in which leveling is possible, as Nightsong mentioned above, are PvE, PvP, and crafting.
ArenaNet has made it clear that they want PvE and PvP to have close to similar leveling patterns in terms of how long it takes to level up. Every aspect of the game is supposed to bring something new to the table. PvP may, for instance, allow players a completely new aspect of a game and different types of rewards for partaking in such content, like the 10 slot bags that so many people have mentioned in this thread and previous ones. Crafting allows for access to anything that the mind desires in terms of material items in the game. PvE also provides access to the dynamic world, nice items from karma vendors, random drops, gathering nodes, and a plethora of other wonderful things. No one aspect of the game is made to negate the other in terms of overall game play.
However, as Nightsong already mentioned, it all depends on your preferences in play style.
I've done PvE a ton in all three BWE's. The game gets signifiantly more challenging as you level up, so there is no "breeze" in game play in PvE later on. It also becomes way less "mundane." What I mean by this is that the more you level up, the less tasks there are (as some people call hearts). As tasks lessen, dynamic events increase.
Also, I've become severely addicted to Tailoring and Jewelcrafting. Dear god. I shall be ahead of the game in terms of knowledge in the crafting system when the game comes out.
Lady Yosei Hecate
Bona Dea Ubume