"... similar to Feral's hybrid specs which Blizzard wanted to remove..."

Wanted to remove, but finally gave up and just made it so that druids have a full-on 4 specs.

Bodes well for this sucking around, I think.

So clearly, the answer is for Blizz to give warlocks a fourth spec; Demonology can stay a DPS spec, and there can be a spin-off from it that's for tanking.

C'mon Blizz; you said you'd do it for other classes where it made sense, and here's a great place to start. (You know you want to.)

Feral is losing that emergency off-tanking ability in Mists (unless you have a specific talent that no one will get), so I guess someone had to be able to take over that role. This might actually mean that demo will be competitive if they're counting on people bringing them.

If I'm understanding it correctly, it looks like the glyph will gimp your dps and make Metamorphosis a tanking form rather than a dps-boost. Whereas the bearcat could switch back and forth within an encounter and perform effectively in either role, a demo warlock will probably have to choose between one of the other to perform effectively. They can switch the glyph between encounters, but dual-spec already allows just about any class to do this anyway so it doesn't seem like the warlock is getting anything too special in this regard.

It's definitely an interesting experiment by Blizzard, with a single glyph having such a huge impact and effectively creating a fourth warlock spec without a fourth tree. It seems like something similar could have been done for feral druids, but with the established expectations being that they can already tank and dps as feral, there's a much bigger downside if that experiment fails and breaks feral tanking or dps for druids. If it fails for warlock tanking, nobody's going to lose their existing role in their guild or raid team and if it succeeds it is a huge win with potential implications for some other class/specs.