I'll take a stab at explaining the rune mechanic. Once you see it in action, it's pretty easy to understand.
Just focus on Blood Runes for the moment. The big change is that rune #2 will never start filling up until rune #1 is full. They always fill 1 then 2. Today 1 and 2 can both fill at the same time.
In Cataclysm, when you're killing things, you use rune 1. Then any extra "red" in rune 2 will fill rune 1 back up again. If both of them are full, you can use 2 Blood Runes immediately. But after that, rune 1 will fill up first and then rune 2. If it helps, imagine rune 2 is the extra tank.
This sounds like it will slow down DK attacks, and it will to a point. That's part of what we're trying to accomplish. We can then fill those extra GCDs with things like free abilities or runic power abilities or we have room to add talents that make runes fill faster. Remember, slow attacks can hit harder though. Instead of DKs hitting fast like a rogue, they'll hit slower and harder, like a warrior, which fits a lot of player's image of a DK anyway. Dual wield will hit faster of course.
I'll try another comparison. Imagine that all rogue abilities cost 100 energy. They have to wait until they get 100 energy, and then immediately use an attack so that they aren't wasting future energy. That's how DKs play now, except they have 6 runes to watch. Now imagine the same rogue except all his abilities cost 50 energy. If he hits an attack when he has 60 energy, then 50 is consumed but he has 10 energy still left and a head start on the next attack. That's the way we want DKs to play.
If that still doesn't make sense, then focus on what the experience will be, which is that you'll have more breathing room in your rotation and won't have to hit a button every single GCD. If you don't use a strike the second it's available, that's more okay because the extra tank will store extra rune resources rather than just wasting it. You'll still be hitting a lot of buttons though. We're keeping double rune strikes and Death Runes and disease multipliers and all of that. We'll have to make some changes in some abilities to accommodate the resource change, but it won't be unrecognizable to you.
We're not sure DKs even need Rune Strike any longer. If it survives, we'll turn it into an instant swing. But if we turn it into an instant swing, then it really isn't that different from existing strikes so it's possible we can just make a tanking rotation without it. Blue poster text.