Prevent disk i/o intensive apps from blocking other apps that only do limited disk i/o. Again, we need some class or hierarchy structure to collect the statistics reliably. Please note: AFAIK current disk io schedulers are only "disk input schedulers".
With the release of the kernel.org 2.6.25 kernel, there is now the cgroup extension available. It has already been integrated with the CPU scheduler, and it could also be possible to use it for memory and/or disk i/o rate distribution between, for example, users.