I think the big thing with dungeons is that (especially at lower levels) they're just to damn hard.... I haven't played a dungeon as a lvl 30 in awhile, so maybe it's changed since then. but at the time it really put me off of the game.
At the low levels, when people aren't used to the game, they're building up there character and developing a play style. When you go into your first dungeon and you're just wiping left and right to AOEs and bosses with no aggro table it really sucks. It makes it feel like everything up to that point has been meaningless.
That's because at the time I didn't know how to play the game... But neither does anybody else in that situation, and then they all get frustrated and quit like I did.
AC Story has been severely nerfed based on feedback exactly like that. It's a bit of a cakewalk now. AC Explorable has become a little tougher for low levels, but it's a more appropriate ramp-up.
For my part, my first few times doing AC Story which involved wiping plenty and then eventually powering through and winning felt incredibly satisfying to me as a successful challenge, and forced me to get good at the core gameplay which helped me through the rest of the game. But it definitely wasn't a gentle curve and I can understand that most players were frustrated by it.