Dungeon hosting
The Dungeon "hosting" system is bar none the worst I've ever encountered in any MMO.
If the person who first entered (i.e. hosted) the dungeon leaves your party, your whole party is kicked. Have no idea why they would implement it this way, except as an interim system that would be scrapped quickly in favor of a real system. Unfortunately, it's been this way for basically the entire life of the game. It's stupid and I don't understand why this isn't a priority for them to fix. I don't believe they have even said anything about this. We know a lot about what their plans are for the future (adding LFG system, balancing WvW/PvP achievements, revamping existing dungeons, adding new tier of profession and armor) but nary a word about dungeon hosting, which is puzzling. It almost makes me believe that this system is here to stay, permanently, but that can't really be the case, because the existing system is so bad.
Clipping
Guild Wars 2, if you haven't heard, doesn't have gear progression like a lot of other MMOs. There's no gear check to enter raid A, where you use drops from raid A to progress to raid B. You can do pretty much all dungeons in just basic quality gear. The "endgame" is focused mostly on looks. The most prestigious, difficult-to-get items - the legendary weapons, aren't any more powerful than the weapons you can get from running a dungeon a few times, but they look really cool. That's why having so many clipping problems in the game is particularly egregious - clipping in this game isn't really any worse than other MMOs, but when reward systems emphasize cosmetics then you should make at least some increased effort to alleviate clipping. Beards/hair/manes are often removed when you put on a helm, and god forbid you play the beast race (charr) because Arenanet artists apparently don't care that you have a tail or horns.
Trading Post
The Trading Post, which is the in-game auction house, lacks basic functionality like being able to filter by armor weight (light/medium/heavy) or even search for backpieces. Through a year in the game I've sold a lot and bought a lot of items and yet you can't search the items you bought or sold, instead you have to go page by page of items and look manually. Additionally, the game offers small and large interface sizes in addition to the normal size, but if you use either of them the
Trading Post becomes incredibly blurry.