Threshold. Pixel and radius are the most useful variables. Pixel determines the extent of the reverse-mask effect on hard edge transitions; try setting it to something like 5 pixels and 500 percent, and you will see a very clear double line with 5 light pixels then 5 dark (or the opposite). If you have detail which is already down to being maybe 4-5 pixels wide (not much texture in an image is any smaller than 3 pixels) then obvious using 5 pixels mask will either ignore such detail or disrupt it.
The threshold setting determines what it does. 0 threshold means that USM will 'see' differences of just 1 step (in 256) between adjacent pixels. Even the grain or noise will be detected as detail, and sharpened. If you want to make grain look strong, simply use 0.5 radius (half a pixel), 0 threshold, and 500 per cent strength.
In most low-ISO images, there is not a significant value change from pixel to pixel within any area of plain tone, so setting 0 threshold is OK. You will get some increase in noise contrast, but not bad. To remove any tendency to get added noise, use higher threshold values - generally, up to 8 will be safe. Beyond 8, setting a higher threshold tells USM to treat noisy or detailed areas as if they are a single smooth tone. It begins to act like noise reduction software. As you increase threshold to higher values like 24, 50 or whatever you will see that noise is replaced by loss of information. The original detail in the picture is replaced by a smoothed tone.
One of the most useful filters in Photoshop is 'Sharpen Edges' which is a special type of USM. It applies a relatively high threshold value and a very small radius edge-effect, smoothing tones while defining high contrast edges. 'Sharpen Edges' is very useful for web pictures, as it can reduce the data size of the compressed JPEG while making the image appear crisp; you can use a higher quality JPEG setting without getting too large a file, especially for avatars and stuff like that.