On the subject of rune swapping, says Ghostwalker, they designed and balanced all the Death Knight skills and talents around the idea of a runeblade with the BBFFUU setup (That is, two of each rune type). While they could allow Death Knights to reconfigure their runes, it might lead to situations where the Death Knight is overpowered or underpowered -- maybe even from battle to battle as they find they need a different set of runes.
Their aim with the Death Knight, however, is flexibility no matter what spec they're in. No one spec for a Death Knight should be demonstrably "better" for DPS or tanking, just different. Forcing Death Knight tanks to go Frost rune heavy to keep up Unbreakable Armor at all times is not their vision for the class. Rather, each tree should be able to tank end-game content with baseline skills and some good tanking gear, with any tanking talents in their build being simply icing on the cake.
The ultimate idea is that even if Death Knights can't customize their runeblades anymore, they can customize their talents in ways that some other classes might find surprising.
Of course, this beta, so it's a perfect time to work these kinks out, discuss this philosophy, and see if it works. I'm still wary of the idea of a class that can tank without any serious mitigation talents or a dedicated spammable high threat move or two myself, but hopefully as Death Knights in the Beta get level 80, they'll be able to better test the possibilities for tanking as an end-game Death Knight.
Ghostcrawler also revealed quite a few tidbits about the mechanics and philosophy behind the new runeforging enchantments. To start with, they do take up the "permanent enchantment" slot on a weapon, and thus won't stack with an enchanter's weapon enchantments.
However, they are meant to be both cheaper and better than an enchanter's enchantments to compensate. For example, the Rune of the Fallen Crusader is a definite upgrade in all aspects to the Crusader enchantment. In addition, Ghostcrawler is looking into providing new runeforging enchantments such as a Mongoose substitute, and is looking into the possibility of increasing the proc rate on the enchantments, since Death Knights don't have abilities like Sweeping Strikes or Sword Specialization to use to get extra procs.
The Blizzard devs do seem pretty dedicated to making sure the new runeforging system provides the best enchantments possible for Death Knights, and are a good way to make runeblades feel customizable and epic without straight up rune swapping. If we can get more powerful enchantments and the ability to do the things in Jadefury's videos with just the 2/2/2 setup, I'd have to say I'm more than satisfied, and looking forward to seeing how the enchantments act out in the world.