What's the difference between the Threshold slider in the USM filter and the Reduce Noise slider in the new Smart Sharpen filter?
Having used Photoshop CC I honestly can't easily compare the two in the way you're asking. They're simply too far apart.
Unsharp Mask adds edge effects to give the illusion of sharpness. Smart Sharpen (CC) now does a quite sophisticated deconvolution and really does make an image sharper.
I CAN say that the Reduce Noise slider seems to work very intuitively and smoothly, and that the quality of the result from the new Smart Sharpen filter is second to none.
I suppose the conceptual similarity is that the Threshold slider allows you to avoids sharpening things like the background level of noise in the image, while more contrasty regions of the image get the full benefit of the sharpening. That appears to be what the Reduce Noise slider does as well. It doesn't really reduce the noise, per se, but just avoids making it stronger.
Here's some additional info, showing an image quite a bit zoomed-in...
No sharpening applied (note Amount: 0%):
With strong sharpening applied, but with 0% Noise Reduction. Note that it has brought up the noise in the green background.
Same level of sharpening as above, but with 20% noise reduction applied. Note that the background has been pretty much left alone, but the subject has been sharpened quite a bit:
After (500%, 1.4 pixels, 20% NR):
Something to try in addition is to convert a raw file to an upsampled resolution, but dial in NO sharpening during the conversion. Then deconvolve the upsampled document in Photoshop with Smart Sharpen. It's possible to wrangle QUITE a bit of detail from a raw file.