Blizzard's certainly shied away from making certain enchants too powerful before. For example, the Crusader enchantment was given diminishing returns after level 60 specifically so it wouldn't overshadow any other weapon enchants. With that in mind, this move does fit in with Blizzard's modus operandi. And if nothing else, it's good that this was nerfed now, before it made its way onto the live servers. While some tanks may not be happy with the outcome, this is certainly an instance where the PTR testing process worked.
Death Knights will still get to keep the +2% stamina on their Rune of the Stoneskin Gargoyle, in part to make up for the lack of a shield and a solid defense sigil. Ghostcrawler also did not rule out the possibility of future tanking enchants, but only if they could find out a way to prevent the same problems that caused them to take out Titanguard.