As far as threat goes several things have been tweaked that allow expertise to be a phenomenal threat stat. Since Reckoning is now a top DPS talent, SotR misses can really hurt threat, and other obvious reasons, expertise is a very desirable stat. Long before intentionally adding a point of Strength you want to get 55 Expertise..
I'm not going to argue against expertise being our best threat stat, besides that I'd like to see numbers on it actually becoming better than before because I suspect people overvalue expertise with the new SotR mechanics. The bulk of our threat comes from SotR and thus, indirectly, holy power. Holy power generates regardless of an avoided CS, and all holy power is returned whenever SotR fails to hit. In the event of a single SotR avoidance you lose one filler worth of threat, which isn't very nice, but not nearly as bad as having it or HotR avoided in 3.x.
When we do get unlucky (roughly an 11% chance with 0 hit and 0 expertise, if I got the numbers right off the top of my head) and get two avoided SotR's in a row we'll be pushing back holy power and see a more considerable threat gimp. Other than that, we get more threat from our white hits with reckoning, but I'm not expecting a "phenomenal increase" due to it solely.
Got carried away a bit there.
Regardless of the above, the point I was making was that "we need to cap this threat stat" is wrong and needed correction.
"Especially considering the "healing tank" spec is viable, you'd be wiser to avoid threat stats rather than actively go and gem for them."
I don't see how exactly B follows from A. Having survivability doesn't really mean you can avoid threat. It's a part of our job usually. I'd argue that having a talent/glyph/rotation setup that promotes extreme survivability does the exact opposite by freeing us to gem threat.
Having survivability doesn't mean you can avoid threat, a spec that severely gimps threat output yet still gives sufficient threat does. Anyone's results may vary, but I'm suspicious it's even possible to have threat problems with a regular spec outside of some AoE pulls on beta, where expertise would be of minimal help. Additionally, why would a survivability spec promote gemming threat? "Overcapping" threat does not do anything other than give the tank slightly more DPS, which isn't high on our list of priorities to say the least, nor does the spec increase survivability enough to warrant giving up any through gemming in my experience in live play.