When trying to validate Guild Wars 2 from the mail “Activate your Guild Wars account” you might encounter this error:
The link you clicked is expired or invalid.
Arenanet has given this answer in their announcement:
“If you’re having trouble verifying your email address, you can play without verifying your email address by clicking “Play” on the Guild Wars 2 Launcher if you already downloaded the client. We’re working to fix email verification, and you can verify your address once we do.” Source: http://en.support.guildwars2.com/