The problem is quantity. It's fun the first few times to realize that you can dodge an ettin's huge hammer attack that does 1/3 of your health. But after the 25th time, you just want to stand there and eat it. It's the same thing that bugs me about raiding in traditional MMOs. I enjoy learning a hard fight, applying the strategy correctly and overcoming the challenge. But doing it over and over again week after week is just irritating. I just want to stand in the fire and collect my fucking loot. I've already proven myself, why the fuck am I still playing this game?
PvP combat is more engaging because it's not formulaic. It just feels different to engage another player and to have to play at the height of your abilities to beat them even after the 1000th time doing it. That's why people still play Counterstrike: the repetitiveness of it is still engaging against a human enemy. Difficult repetition against an AI is just fucking annoying.
This sounds at first a bit cray cray, but I get where you're coming from. I think this is more of a rant about the nature of MMOs than anything else. When reviewing other forms of games, reviewers will give negative marks for repetition, applauding a change in formula that puts players on their toes. Intelligent AI is always praised as result. This is also why as the market for gaming grew in the last decade, people unfamiliar with RPGs said "why would I want to press the same button over and over with no challenge?" and as result multiplayer gaming became more popular. Playing against humans adds variety. Their tactics differ, difficulty is not steady. You can play against idiots and you can play against great players - who are still capable of messing up thus adding variety to the mix.
The nature of the MMO is that of the traditional RPG where you are basically doing the same damn thing over and over. The one thing I loved about GW1 was the deathmatch nature of PVP. Tired of the game? Jump into PVP immediately! I still haven't beaten the Prophecies campaign yet have probably sunked tons and tons of hours into that game.
It seems your complaint though is that if a game is asking you to do the same damn thing over and over, at least make it easy. Which on one level is ridiculous. You're asking for the game to be mind-numbing, simple and basically a chore for you to run in the background while you multitask. On the other hand, having to roll around as an enemy attacks you is fun. But doing it over and over again is the same as if the enemy were easy. You essentially are making the mundane chore now harder. It is the grind without the ability to multitask - peppered in with some frustration. I personally think the difficulty issues with the game are not due to the demanding combat system but because the game enjoys spawning guys on your head.
The positive side of the Guild Wars franchise is that if PVE bores you, PVP is heavily supported by devs and considered by many to be the end game. So if you become self-aware of the grind promoted by the game you can venture off to do something else.
Example of how this game's PVE is: Mass Effect 3's multiplayer. Players team up against waves of enemy AI. Combat requires full attention from players, so you can't just run off and make a sandwich. To unlock weapons/characters you need to replay the same levels, fighting the same guys over and over. Players eventually found certain strategies that would ensure a win. Came to the point that I would enter a match, hide behind cover and do the same thing over and over and over again, just to unlock a weapon. I stopped playing once I realized unlocking that weapon was pointless because it would lead to me just doing the same thing over and over again...just a bit faster.
I like my PvE MMO combat to be slow and boring so I can watch videos on my other monitor, zone out listening to music or talk to people on Teamspeak.
This reminds me of playing Star Wars Galaxies...it supported in-game macros. I would run out to a spawn of enemies, hit the macro, and start watching movies. Video games are a job.