I think the real problem still to be solved is responsiveness under I/O as in https://bugzilla.kernel.org/show_bug.cgi?id=12309 - I find that large apps such as VMware and Firefox can cause keyboard input to freeze for many seconds using kernel 2.6.24 and Ubuntu 8.04, which should be a reliable combination by now.
Probably it's a VM issue, but it's hard to understand why Linux can't keep up with my typing these days given much faster hardware, when it always used to back in the days of formatting floppy disks in background.
Firefox is a key issue - with 100-200 tabs this freezing of keyboard input across the whole system is much more frequent, and "pidstat -d 5" shows that it and kjournald are doing some I/O, though at quite low rates. This is on a Core 2 Duo system with 4 GB RAM and PAE enabled, and about 50% of RAM used by programs typically.
Linux always used to be stunningly fast and responsive compared to Windows - with bugs like this, and the choice of a lumbering Firefox vs. a Chrome that lacks key extensions, it's still not quite there as a desktop. My main desktop is still Linux but I'd really like it to perform more consistently.