Just to make our intent clear, the Glyph of Demon Hunting isn't intended to turn Demonology warlocks into a tanking spec. You won't be able to queue as a tank for Dungeon Finder for instance and won't have the survivability or tools of say a Protection paladin.
Historically, warlocks felt tankier than other casters and could even off-tank some encounters. We have made an effort in Mists to recapture some of that flavor. A warlock with this glyph should feel like they are about as effective tanking as an Arms warrior who pulls out a shield and swaps to Defensive Stance, or a Feral druid who goes into Bear form. You might be able to off-tank adds or pick up an actual boss for a short period of time if the tank goes down.
To make warlocks an actual tank would take more significant changes. For example, we want tanks to have to pick up separate tanking gear than their DPS gear (this is even true of druids) and want tanks to have to give up some of their DPS potential in exchange for their survivability. In short, it needs to be a commitment, and that's the sort of thing that needs larger gameplay changes than just a glyph.
That shouldn't stop the glyph of Demon Hunting from being fun though. As you can probably tell, we are trying to make even the major glyphs more about character customization and fun.