Most notable have been the extensive
SMP enhancements and work to provide fine-grained
multi-threading/
locking in the
network stack. This was spurred, even
Linus admits, in response to the
Mindcraft survey, finding
NT running
IIS on a multi-CPU machine serving
static content faster/more
scalable than
Linux running
Apache.
As some of the more informed laymen might imagine, this has involved rewriting a fairly significant amount of the kernel, and has been a monumental effort, taking much longer than folks guessed. Of course, the reason why you want to run Linux, faster than NT or not, is because unlike Microsoft, the linux guys won't release it 'till it's finished. It's nice not to have to reboot every day to keep from crashing.
Anecdotally, the enhancements have been extremely effective. Especially anyone with a multi-CPU machine should find the new kernel well worth the time to upgrade.