The new graphics driver and chipset support seems pretty important for the average user (given that they have the the respective hardware). Apart from that, optimizations such as lock removals might make some things a little bit faster.

Edit: and the power saving for hda_intel audio probably will affect a lot of people as well.

commit 442b9635, increasing the initial tcp window (initcwnd) to 10(segments), first landed somewhere around 2.6.39-rc2. I consider that a significant thing for servers. The initial receive window was increased first, several months ago. (I think the receive window change was in .37 but I don't know for sure.)

I consider the inclusion of ipset to be a big deal. It certainly makes things easier and faster for people who do whitelisting, or have large block lists on servers. There are many times I've thought "I sure wouldn't mind using ipset here" only to decide it wasn't worth it due to the pain of compiling it myself and getting it to work with any given distro. Now that it is stock-kernel, the pain goes away. I'd expect to see it start cropping up in all sorts of places (e.g. torrent block lists)

There's a patch in there that means the RCU filesystem optimisations from 2.6.38 are now operative for you if you run a kernel with security modules like SELinux active.

The LWN article on 2.6.39-rc1 had the main details: http://lwn.net/Articles/435908/

The most significant to me is that ipset is merged into main branch.