-Threat

How exactly do moves with -threat rating work? Any differences between using them solo and teamed? I'm assuming it works differently than an actual placate effect, right?

"Threat" is what the AI uses to decide whot o attack. You actual Thrat value is a combination of your Threat AT modifier, how much damage and debuff you're dealing, the specific AI you're dealing with, how far away you are, and 1,000 times the remaining Taunt duration you have on the AI (that's correct, Taunt is not binary, and longer-duration Taunts are stronger).

There are also other factors we don't know about in the Threat calculation, and we don't know exactly where Threat gets floored (probably at 1, but we just don't know). I believe when Castle told us the portion of the calculation we know about, he also mentioned there's a "KillTheHealer" variable which is currently being ignored by all AIs in the game.

In short: Solo, -Threat does nothing (you're the only target to attack), though since most -Threat powers come alongside stealth powers, you may be ignored due to the stealth component. On a team, -Threat makes it less liekly that the AIs will target you.