Full disclosure - I hate the DH class, but always try to be impartial and objective in things, so here's my two cents.
What counts as a failure? Plenty people play it, so it's no a failure on that front. If you assume there's only a small number of deck arcehtypes (control, aggro, tempo, combo) there's no reason to think that it would (or should) be different from those. The issue in this regard is that the only viable deck appears to be tempo (but is that any different from priest who can only really play control?).
Where I think it does fall down (aside from the OP cards that I still think remain) is that the random generated cards might be too predictable. Take Cobalt Spellkin - he only has a choice of 6 spells which gives a pretty good chance of getting whatever you were looking for. Compare that with mage who has 12 one mana cards (discounting quests) in standard and you can see where the issue lies. Aside from SHaman (who only has 7 one mana spells in standard) all the other classes have much more variability.
I think this limited deck choice create the main issue with the class. They just don't have enough cards to flesh out some other decent decks. I would love for "big demon" DH to be a thing, but it isn't. There just aren't enough cards to make it work.
I do think the one mana hero power might problematic in the future though. Its fine balance wise compared to all the others, but it probably pushes the class too far towards tempo based decks in the same way that the hunter hero power pushes decks towards the aggro/face decks. That in itself might limit the options for the class.