First off is something that I really like, full USB support. Yes USB had been backported to 2.2.x, but it still requires patching the sources AFAIK. It's good to see this in the kernel tree. I hate having to switch my mouse back to PS/2 when I boot Slackware instead of Win98 or FreeBSD. [being a *BSD advocate, I will mention that FreeBSD, OpenBSD and NetBSD have had USB support for at least a year. :-) ]
Next comes the built-in firewalling. iptables works much like ipchains and ipmasq used with previous kernels. The bonus comes from having the packet filtering and masqerading done in kernel space, for a speed increase. People operating Linux based firewall have been looking forward to this.
Another speed increase (at least on multi-processor machines) comes from improved SMP (symetric multi-processing) Linux can take advantage of multiple processors better and faster than ever. On the same line is multi-threaded TCP/IP, allowing IP connections to be handled more effeciently and quickly.
DRI (Direct Rendering Interface) support is a huge thing for anyone running X or playing games on Linux. Previously, all graphics stuff was done in userland, which is also why many SVGAlib based games had to be run suid root. With graphics drivers now in kernel-space, we will see big speed increases and greater support for the advanced features in modern graphics cards.
As one K5er in #kuro5hin said, "tons of stuff" is new and improved. Speed increases, bug fixed, more hardware support, CPU optimizations, the list goes on.
The only question left for me is: "When will Patrick Volkerding deem 2.4.0 and it associated tools good enough for Slackware?" :-)