Kernel Interactivity and Responsiveness
One of the areas of focus for Linux 2.6 has been in making the system more responsive for desktop users and other uses that need a high degree of control over when events happen. Each of these target uses have very different challenges, but there have been many changes that can benefit both.
On major internal change present in the 2.6 kernel that should not be understated is that the kernel itself is now preemptible. In all previous versions of Linux, when the OS is currently doing something in the kernel, it can't be interrupted (and on multi-processor machines, this was true on a per-CPU basis.) As of Linux 2.6, the kernel now allows itself to be interrupted mid-task so that user applications can continue to run even if the kernel is doing something complicated. (In order to avoid the obvious race conditions that this can cause, the kernel does have certain sections of the code locked so that they can not be interrupted while in progress.) The primary benefit of this change is that interactive performance (for desktop users, for example) has been given a boost and so the system will "feel" faster for things like user input.
One other change that will help make Linux into a more responsive operating system with applications that support it is the inclusion of support for new "futexes" (or "Fast User-Space Mutexes", if that helps.) Futexes are a way in which multiple processes or threads can serialize events so that they don't trample on each other (a "race condition").
>>The Wonderful World of Linux 2.6
For more information see: Slashdot: Linux Test Kernel 2.6 released.