I played through the Ascalonian Catacombs yesterday. I have to say, that was definitely the least fun I've had playing this game.
There are two ways to design a boss. One option is creating a unique enemy with its own mechanics and patterns, forcing the players to adapt their playstyle and take advantages of the boss' weaknesses to defeat it. The other option is to take a normal enemy and pump up its stats a bunch. This option is the most uninspired, lazy, and uninteresting way to make a boss (see: Skyrim).
I've seen dungeon bosses in WoW. The warriors spam all their attacks and health potions as fast as possible, the mages spam all their spells and mana potions as fast as possible, and the clerics spam all their heals and mana potions as fast as possible. This goes on for about five minutes and then the boss dies. These "war of attrition" bosses actually work pretty well in WoW, it would seem. Unfortunately for GW2, the mechanics of their game are pretty radically different. There are no health potions. There are no mana potions. There are no clerics. This makes these kind of boss fights doubly frustrating.
So I was supremely disappointed to find out that almost every boss in the Ascalonian Catacombs was an uninspiring, super tanky war of attrition type boss with no real strategy required to beat it besides "don't die". Most random events have more nuance than this. The most interesting thing most of the bosses did was summoning minions.
There was one exception. The necromancer boss has a neat mode where she's powered up while some minions dance around her, and you have to kill those before you focus her. This was fun. We had to switch our focus when this happened, and even that little bit of strategizing made this boss fight a lot more dynamic and interesting than any of the others.
There was one that was almost an exception, but still managed to fail. The lovers are a pair of bosses that are more powerful near each other. How do they get powered up? More tankiness. Seriously the most boring way you could possibly make a paired boss like this. They didn't even bother to give them a combination attack or anything.
Another thing that ruined this fight is how stupidly difficult it is to get the two mage bosses apart from each other. They're both ranged, so they don't really need to move much to attack you. So get this: the bosses whose entire strategy is based on getting them to move around don't move much. This could have been pretty easily solved by, say, making one of them melee. Or maybe just making the bosses more mobile.
I'm imagining it now. A thief/ranger combo boss. They're both very agile, jumping around a lot. Your goal isn't to lure them apart and hit them for five minutes, it's to coordinate your attacks for when the bosses naturally move apart. This theoretical boss fight is already way more interesting and I haven't even thought up anything cool for them to do when they're together.
tl;dr: Basically this dungeon was four boss fights that were basically "I am tanky and deal AOE damage" and one that was only slightly more interesting than that. This is my biggest gripe with MMO games. They tend to get the idea that they can be lazy with design since you'll care less since you're playing with your friends. GW2 has dodged this bullet in a lot of other areas, but I hate to see it happening here.