NHE, thanks for working on this for us. I think the developers of Prosilver should have consulted the disabled, especially the visually impaired, before front-end design changes were made. From what I can tell, the changes are mostly cosmetic, but here's my two cents:
I prefer Subsilver2 because I like corners on boxes to be old-fashioned and square (not rounded which appears childish to me) and the avatars to be on the left (not on the right) which is where my eyes naturally and automatically go to start to read text--it's hard for my eyes to track over to the right to see the avatar and read the user info and then have to track back to the left to start to read text. Also, I prefer the thread icons in Subsilver2 because in Prosilver, the icons with the vertically scrolling lines are very distracting by causing my vision to start flickering.
If Prosilver becomes the default setting, in order for a user to keep using Subsilver2, the user must change his or her settings every time the user logs in, but I and my eyes think using Subsilver2 is worth this bit of extra effort.
At some point, though, developers will stop supporting the older Subsilver2 version, so a switch will have to be made eventually.