As some of you have pointed out, the glyph of Demon Hunting in your beta build doesn't deliver on what I described above as the design. The newer version of the glyph, hopefully available in the next beta build, allows a warlock to toggle in and out of the off-tank / survival mode, more like the druid going Bear or the warrior going Defensive.
The new shapeshift granted by the glyph is called Threatening Presence (PH) and adds Taunt and Fury Ward to the normal Metamorphosis toolkit. You still get the damage reduction from mastery instead of the damage boost from mastery while in this form. You can also choose to go into normal Metamorphosis even if you have the glyph. Let us know how it feels.
We appreciate all the feedback and passion on this topic. We expected this glyph would serve a pretty niche community and were surprised at how many players were excited about it. We are still in beta and are still very much experimenting with what kind of gamplay we can deliver in glyphs.