Plug 'n Play and the PC Speaker driver definitely come at the top of my wish
list for 2.2. The latter is stable on 2.1.59-VGER-CVS971020 with SMP, at least
here. The former is a bit too fundamental to be included at this stage, so I
suppose it'll be 2.3, unfortunately. I expected it in 2.1 a long time ago, but
was disappointed.
Then there's NFS, NTFS, Joliet, FAT32.
Oh, and Michael Chastain's Makefile patches to make CONFIG_SMP a proper config
option.