To be perfectly honest with you, I don't actually know how the threshold setting on usm works. Increasing it seems to soften the sharpening effect, but only in the same way that decreasing the 'amount' slider does.
I'm sure someone with deeper technical knowledge of PS will be able to explain the subtle difference, but I can't see it on screen.
Consequently I tend to leave it set to '0' and just use the 'amount' and 'pixels' settings. Works for me.
Obviously the contrast, saturation and sharpening on the above photos are to personal taste, and can be increased if preferred. I find that subtle changes work better though, too much adjustment can look overprocessed and too 'digital'.