Dedicated servers are one thing that is expected by most PC players coming to a multiplayer game. It's through these custom rulesets and game modifications that a lot of today's top developers first learned their skills, after all, and it's a big part of PC gaming history. This is why those who play shooters on the PC raise such a big fuss when console ports don't ship with dedicated server support.
For MMOs, though, it's a different matter. The fact that everyone must compete on a level playing field in both PvE and PvP play means that standardization from the developer is required. It doesn't completely stop cheaters and griefers, but it does go a long way to dealing with the potential problems, at the expense of customization.
Guild Wars 2, like its predecessor, does things a little differently when it comes to PvP. It's not necessary for a player to battle their way through the PvE campaign if they want to jump into competitive play, as it's possible to create a PvP-only character who is already buffed up to an appropriate level. Playing the PvE campaign unlocks additional skills and customization options, but theoretically it's possible for a new player to compete on a relatively even footing with someone who's been playing for months.
MMORPG reports that because of the fact the game has PvP exclusive characters which don't affect PvE play, ArenaNet has decided to hand some degree of control over to the community. It'll apparently be possible for players to create and host their own servers in order to play with specific rule sets. By extension, this opens the door to the possibility of game modifications, custom characters, new maps and all manner of other goodness to keep the competitive play fresh long after the game's release.
ArenaNet hasn't confirmed any specific details as yet, but watch out for more information on this exciting possibility soon.