I think ultimately you just have to use your head about the Unholy rune especially, I don't think there is a good rule of thumb beyond the one you wrote as well as the stated priority in the OP. I suppose that in AoE scenarios it has slightly clearer use given Death & Decay, but on the other hand you might easily be in CD anyway.

As Consider outlined in his OP, it's really not that big of a problem. We simply have to reforge all the hit off the gear into mastery and gem it instead. This will result in the exact same stat distribution, just with hit being in gems and mastery being on gear, instead of the other way around.

While Mastery really is as simple as you make it out to be, Haste isn't, largely because of the softcap. Yes, it may affect ~90% of our damage but not from 0 to infinity rating, and not for 100% of the fight. Mastery, on the other hand, may only affect 50% of our damage but it does so equally from 0 to infinity and without any pause or exception.

What the soft-cap is, exactly, is still something which needs to be determined (I haven't yet figured out the best way to go about it), but even without knowing it exactly, we do know some:

First, that the rune regeneration aspect of Haste is worthless (or, at best, only marginally noticeable) under the affects of Bloodthirst/Heroism/Time Warp. Whatever the cap may be, it's definitely under 30%. Heroism (et al) last for 40 seconds, which is ~13% uptime on your average 5 minute fight (and much, much higher on the current ICC fights!) - that alone causes haste to take a pretty decent hit in worth.

Secondly, there's speed pots, which similarly cause haste to greatly devalue for their duration. When you consider that you prepot one and use another mid-combat, that's a solid ~20 seconds where all you're getting from your haste is attack speed. That's another ~7% of your normal fight length.

Those two facts alone, using rough napkin math, would place the two stats equal to each other: 0.5/90 verse (0.8*0.9)/128. The hits just keep going for haste, however.

Then there's the fact that after hit/expertise, haste is still our second best secondary stat which means that even if we don't go out of our way to stack it, we're still inevitably going to have a sizable amount considering our most optimal itemization (in a vaccuum) would be mastery/haste (and, currently, crit to haste with the crit all reforged to mastery).

There's dozens of other factors, too. Troll racial, engineering enchant, herbalism cooldown, trinket procs, so on and so forth. Ultimately it all just goes back to the soft-cap (which I know enough to say isn't very high and a point we can currently reach with relative ease, it's not worth then stacking additional haste).

At 85, I don't see things looking that differently with the conversion ratios being as low as they currently are. They really need to be jacked up, doubled or ideally tripled, but that's another topic entirely.

Also, it's worth noting that the combat ratings thread is mistaken. We no longer get 30% more attack speed from haste than other classes, not on beta at 85. 128 rating is 1% rune regen and 1% attack speed, for us or anyone else.

It's worth noting that KM no longer affects HB (at least as it is currently shown on wowhead and MMOC). Additionally, it seems like there should be some sort of target cap for when Pest becomes viable. Obviously for 2-3 targets tabbing through with PS will do more damage, but on a fight like Anub or Freya or BQL Pest'ing diseases out to 4-5 targets would probably be a gain over a single Blood Strike (unless disease damage has been nerfed something fierce and nobody told me).