IMHO, standardized abbreviations would be more intuitive when looking through the listings. WinNT, Win2K, Win9x, MSDOS (or M$DOS :) catch the eye of those who are familiar with that ilk much more readily than W1, W2, etc.

I like the '+' indicator for experience. Limit it to \+{1,3}, as (Beginner, Regular User, Guru) is probably fine grained enough for this purpose.

Thus: WinNT+++, Linux++, VMS+

Just my $.02 worth.