I recently got my 69 year old father to sign up for a F2P account. He's computer literate but not game savvy. After a month of casual play and much tutelage, he is beginning to learn the ropes. Watching him play, I'd have to say that at least low-level content is tuned just about perfectly for the casual or new player.
He's mostly been puttering about Korthos trying to figure the basic mechanics of running his character. He dies every once in a while but can handle most content on normal. Granted I've twinked his character so that failure is almost an option, but he's having fun and not getting discouraged when he fails a quest.
I suppose the greatest challenge he faces is simply the scope of the game. There is so much to do, so many lessons to learn. What I now take for granted, he has yet to master or even encounter. DDO has a high barrier to entry for a new or casual player. Not because the quests themselves are terribly difficult, but rather that the game is so enormous it can be overwhelming.