While everyone posting here is missing the skill system, and who can blame them, Guild Wars 1 had the best skill system ever, I want to add something slightly different:
Mob skillbars. In Guild Wars 1 all mobs were using the exact same skills available to players, same attack speed, same moving speed, ~same damage (at same levels), ~same healing etc, in Guild Wars 2, just like in most MMORPGs, and even lots of single player RPGs as well, the mobs are using some different skills, they have more hit points, FAR reduced attack speed and more damage (for example Veterans/Champions). Also, there was full skill synergy in Guild Wars 1, the combo system is very lacking in Guild Wars 2, they need to expand upon it. It is important to note that in Guild Wars 1 you didn’t have a 8-skill skillbar, you had 64, 8 per character in a 8-man team with FULL synergy among them, it’s wrong to say that in Guild Wars 1 you had just 8 skills. In Guild Wars 2 there is absolutely no party building required.
The system in Guild Wars 1 allowed Arenanet to create challenging encounters without adding 1-shot kill abilities, super damage red circles, without increasing damage, without increasing hit points. Instead, all they had to do to make an encounter difficult was to add “meta” skillbars on mobs. Just like what they did in Slaver’s Exile or any of the Guild Wars Beyond content, “pro” builds so players were occasionally fighting mirror builds of themselves.
The system also allowed the inclusion of pve-only skills, that “forced” players to earn reputation points to make them more powerful. Some of those pve-only skills were seriously overpowered and could tip the balance when fighting in Hard Mode. And so we come to the second missing point in Guild Wars 2:
Reputation. The various factions in the game were giving benefits when fighting in their respective zones, while also improving the power of certain pve-only skills. The reputation system increased the replayability, depth and of course longevity of the game. It tied in perfectly with Hard Mode, Vanquishing and finishing those Handbooks for reputation, which required the player to complete either all story steps or all dungeons, or at least a great number of them.
They removed that perfect system and instead on focusing on doing ALL dungeons players are “forced” by the system to farm a single dungeon for the looks/gear they want (how lame is that?) unless they want the Dungeon Master’s title. In Guild Wars 1 there was good reason to do all dungeons and repeat ALL missions, something that is solely missing in Guild Wars 2 were players are required to jump farm/grind specific places.
Secondary objectives, all missions had some kind of a secondary or optional objective. Those added an extra layer of depth, difficulty and replayability. Also, in a game with full npc teams, they made it essential for players to group up and do content together, who remembers the mission in the jungle or the desert in Prophecies? Some of the secondaries were extremely hard with henchmen (not so with heroes though)… These objectives gave an extra, important incentive to go back and redo missions.
Also, very recently I created a Charr and played with a group of friends up to the level 10 personal story quest. Let me tell you that a group of 5 has a different experience than soloing, Anet did an excellent job at scaling the personal story quests with more players but nobody will see it, nobody will see how epic Battle of Claw Island is with 5 players, nobody will fight the Mouth of Zhaitan with 5 players, that has little in diffirence than a dungeon boss etc. There is potential to turn some PS quests in missions, like in Guild Wars 1, along with secondary objectives, replayability, why not add Hard Mode and reputation books to them as well.