Wife and I did one of the solo-only instances yesterday. It was the Mages Guild quest where you face the big rat dude at the end of some mansion. Found it pretty easy with my NB, but my wife's DK was getting her rear end handed to her.
At my wife's request, took control of her character and completed what she could not. Still, it would have been preferable to simply be able to cooperatively complete the instance together.
Sorry to say that, but blame bad player, not a game ...If you could finish this quest playing your wifey character, then the problem is not with game, it is with your wife(just example) and other players lack of skills...