IMO - priority #1 needs to be consistent site performance, whether it be 10 seconds to load a page, or 3 seconds to load a page (based on a broadband connection). I think that people would tend to get used to whatever speed it is as long as it is consistent.
After that I would say whichever programs run well under both Vista and XP would have the most exposure in the general user base, so galleries that cater to those programs would be good.
I would also recommend creating a "Widescreen" wallpaper section (or easy link inside of wallpaper library), due to the increasing popularity of these monitor screens, which will probably increase over the next year. This may invite artists to create more of them as well.
As for "WinAmp", I believe this is still one of the major players, so default accessibility would probably receive traffic (if this is desirable based on cost of said traffic vs. resources to pay for it).
Gadgets (for Vista) will probably play into the future, as long as they are tested and of high quality and variety.
The forum page (being overhauled and / or redesigned) is a must for community support.
Assuming WindowBlinds, IconPackager, ObjectDock, and ObjectBar are all fully compatible with Vista (final release) - I can see value added content being important to users in these libraries. I did not see anything in the Vista Beta 2 that would preclude these programs doing well (assuming full compatibility
).
Again, the consistency of the site performance would be of great importance for "word of mouth" advertising.