to much to list, I suggest going to www.guildwars.com, clicking on the link to updates and the reading them, including the archives, oldest to most recent.
well, what are the biggest changes? I'm talking more along the lines of game changes...not so much like "Well, Sword skills were all nerfed. But fire magic was upgraded!"
More like "They added new types of quests" etc..
lots of new creatures added, in GW Factions... not much has changed in vanilla but the balancing... unless you count removal of the book/gear effect, aoe nerf, tombs being a portal to hell now, the rank 15 kangaroo emote and more.