Since we've only seen 3 of the 5 races so far, I can only assume they are still working hard on them, along with polishing up the higher level areas the Beta couldn't reach. They are likely also still fine tuning the games balance and performance.
First Public Beta when we were in larger numbers could barely see our opponents render in WvWvW until we were right on top of them, which seemed to be fixed by the next Public Beta. Last Public Beta (3), there seemed to be a problem with how servers were matched against one another leading to very one sided fights that Arenanet will no doubt be trying to have fixed by the next Beta. It seems in computer games there is always another problem to solve, and solving one problem seems to always open up new problems. I've no doubt that even with the games amazing polish, Arenanet still probably have their hands full.
Plus Arenanet are living in the shadows of both disastrous starts like Warhammer Online and Age of Conan, and magnificent starts like Rift. Rift in particular has set a very high standard of polish - I am comforted to think that Arenanet might be trying to follow Rifts example in making Guild Wars 2 as polished as possible for launch.
On top of all that, the conspiracy theorist in me can't help but wonder if Guild Wars 2 has a few as yet unseen and untested new surprises in store for the game that they have yet to unveil.
