For video-nerds, 2.6 incorporates DVB (Digital Video Broadcasting) support, which means you can build your own Tivo or ReplayTV-type device. Imagine- owning one of these without spyware!
Do I Upgrade?
Short answer: Don't be silly. Like all major kernel releases, 2.6 is mostly backwards-compatible, but there are some applications that will not work with the new kernel, without tweaking and patching. The wise admin tests it thoroughly first. One way is to install it on any machine that has the applications you'll be running on it, right alongside the existing kernel, then select the one you want at boot. Part 2 will tell how to do this.