I've gone back and forth on whether I think Demon Hunters are a viable class, and right now I would say no.
I've always been of the opinion that demon hunters would never be playable, and MoP has more or less cemented that opinion. Could
they be playable? That's a different question, and the answer is yes, with enough work.
But as far as I can see, the amount of work Blizzard would have to put in to making demon hunters a serious class combined with the potential pitfalls just outweighs the possible reward. Demon hunters are victims of two things - niche saturation and overstylization.
For the first - take a warlock. Take a rogue. Blend until smooth. That's sort of what a demon hunter is - but if you make them too much like rogues or too much like warlocks, they'll feel too derivative. Moreover, demon hunters are dual-wielding melee fighters - just like rogues, fury warriors, frost dks, monks, and enhancement shaman. Essentially, they've been crowded out of their niche.
This is especially true with the modern warlock - go demo, take grimoire of sacrifice, pick up the glyph of demon hunting. There. Now you have a supercharged demonic form (and a half-and-half form) that's a pseudo-melee fighter, hopping around blasting people with shadow and fel. You are effectively doing everything a demon hunter would do. And you can't just yank this away for the sake of a new class - warlock players would howl
and rightly so.
What about their abilities? Demon hunters have long since had their WC3 reportoire (mana burn, immolate, evasion, metamorph) given out, and unlike death knights that had a slew of unused necromantic themes to choose from, a lot of stuff that demon hunters might have once used has been passed off to warlocks, or maybe rogues. Coming up with a new class necessitates a new roster of abilities, but where do demon hunters go that doesn't leave them feeling like a Frankensteinian rehash of other classes?
The second problem is overstylization. Think of every demon hunter in the game. What to they look like? Kilt, double warglaives, bare chest (except for that one chick...figures) and blindfold. So, how do you make this work for gearing? Does every helm incorporate a blindfold and every chestpiece vanish? That's gonna get old, fast, and unless you limit the helms demon hunters wear, that's a lot of work remaking old helms. Do demon hunters use maces? Axes?
The thing about death knights and monks is that "evil knight wearing evil armor" and "kung fu master" are fairly broad generalizations that encompass a lot of potential appearances. Demon hunters don't - they're instantly recognizable thanks to their style, but if you start taking that away for the sake of personalization or gearing, you run the serious risk of taking away the style that makes people like them.
Also what armor would they wear? Another leather class? Hnnngh. We already have 3. Cloth? Agility
cloth - is Blizz really going to throw in another armor type in a game that already has 9? (Str/stam/int plate, agi/int leather, agi/int mail, int/spr cloth, and I'm simplifying here - remember how Blizzard eventually threw their hands up and made spirit->hit for caster druids, shaman and priests so that they could trim down bloated loot tables.)
Again, let me say- demon hunters could
be a playable class, if
Blizzard is willing to put in the huge amounts of work, tackle serious thematic questions, and dare the potential pitfalls in making them. But I don't think Blizzard wants to take the chance of ruining the demon hunters' appeal or running the risk of demoralising players of current classes when they see demon hunters getting awesome stuff that they think should have gone to their classes. It's an ugly potential tangle, and I think Blizzard is smart enough to avoid it.