They could do any number of things to balance this, like making haste reduce the GCD for rune abilities.

Maybe, but Blizzard thus far as been really careful to not reduce the melee ability GCD because they don't want people to be good just because they can hit a sequence of buttons super fast (casters at least have breaks in between a series of shortened instant casts).

Plus, that means a "haste cap" for melee becomes a reality and likely would happen soon into Cataclysm (haste affecting GCD too little makes it an ineffective stat, affecting too much hits ceiling quickly).

Yeah, seeing as how I'm already hitting a button on every GCD the only thing I can really see them doing with haste is make me hit hit them faster.

That honestly doesn't sound to appealing.

There could be a lot of impact on Blood, actually. With a faster rune refresh, it's entirely possible that there could be some situations where blood runes refresh before you're entirely through your rotation, allowing extra heart strikes with a refresh in the middle, or making GoD much more useful, since you'd not be wasting extra GCDs putting diseases up when you could be hitting things.

The ramifications are pretty big, but I don't think we'll see how big until 4.0 drops. Think of it this way: Have you ever run yourself out of runes and RP at the beginning of a fight or after a movement phase? Not any more! Because those runes that are 1sec from being back will have been back for a while now. Go Haste!

With the new Haste (faster rune refreshing, shortened GCD), at least one advantage is clear: the tighter rotations of Glyph of Disease specs (both Blood and Frost) will be a lot easier to maintain. In the current system, you are GCD capped and barely able to execute your full rotation within the disease timers, but with a little haste, you'll be able to shave 1 or 2 seconds off that rotation and have a lot more leeway for missed/dodged attacks or time off target due to movement, or tossing an extra DC/IT/HB. It doesn't seem like it will be a large dps increase on paper, but it will probably be more significant in practice than it looks.

I'll give a nod to bitterfig's point, but I still see Haste being a rather undesirable stat after a fairly minimal level for Blood. Initial rotation deadspace is simply cause by a lack of RP, and stacking haste to minimize that space in an initial rotation is a waste of itemization. The only time you care is on trash (yawn).

Haste will heavily favor GCD unbound specs like Unholy. Blood will probably see a haste cap at a really low rating level. Blood DKs stacking haste will probably have excessive RP rot.