In all the honesty, after you get the general "flow" of gameplay in dungeons, they become very easy even on explorable modes in general. There are some exceptions to this, like Giganticus Lupicus and the WoW-esque tactical approach he requires which feels completely out of place in GW2's gameplay's flow but in general content is quite easy once you get how things work on a general level. Even a completely new instance is generally not terribly hard to do after you get a party of people who understand how things work on a general level, even if you're all first-timers.
This is great for when doing dungeons for the first few times. After that, the dungeons become mindnumbingly grindy due to being too easy with only moments of challenge being the rather frustrating out-of place bosses and mobs that have either bugged or daftly overtuned mechanics. There is very little in terms of "they got it right with difficulty" moments in GW2 dungeons at the moment, where difficulty is challenging but not reliant on a single overtuned trick (like the blowback boss in CM exp). Perhaps I just hit the nail on the head back in spring when talking about GW2's gameplay model being simply unsuitable for challenging dungeon content or perhaps designers just couldn't realise the potential of it. Either way, it's just not that fun by the MMO measuring stick of "needs to be interesting and/or rewarding after first couple of runs".