The purpose of this page is tracking and explaining the features added in every release, just like the [http://wiki.dragonflybsd.org/index.php/DragonFly_Status Dragonfly people did].
It'd be nice if kernel hackers would spend some minutes adding their stuff here. The one place where you can find a comparable changelog are the fabulous LWN kernel articles: http://lwn.net/Kernel/, or the [http://lwn.net/Articles/driver-porting/ driver porting guide] - but there's no reason why the kernel community shouldn't embrace and extend those efforts 
TODO:
Import (and extend, explain, etc) LWN's [http://lwn.net/Articles/2.6-kernel-api/ list of 2.6 API changes] (need to ask for permission?)
Import [http://kernelnewbies.org/status/latest.html Kernelnewbies status] list
Import relevant data from [http://www.codemonkey.org.uk/post-halloween-2.5.txt davej's post halloween document]
- Keep track of what gets changed
2.6.14:
2.6.13:
- x86 now uses the generic PCI bus setup code for assigning unassigned resources
[http://lwn.net/Articles/104343/ inotify]
- Support for the Xtensa architecture
[http://lwn.net/Articles/108595/ kexec and kdump]
[http://lwn.net/Articles/135472/ Execute-in-place support]
[http://lwn.net/Articles/145973/ build-time configurable clock interrupt frequency]
[http://lwn.net/Articles/143474/ Improved CFQ IO scheduler]
- Voluntary preemption patches
- Removal of the devfs configuration option
2.6.12:
[http://lwn.net/Articles/140164/ API changes]
- New driver for the "trusted computing" (TPM) crap^Wchip
[http://www.superh.com/products/shyway.htm SuperHyway bus support]
- Multilevel security implementation for SELinux
[http://lwn.net/Articles/124703/ device mapper multipath support]
[http://lwn.net/Articles/121845/ Address space randomization]
- Restore the Philips webcam driver
- I/O barrier support for serial ATA drives
[http://lwn.net/Articles/134460/ "resource limits"]
[http://lwn.net/Articles/127936/ cpusets]
- Remove IPV6 "experimental" status
2.6.11:
2.6.10:
2.6.9:
2.6.8:
2.6.7:
2.6.6:
2.6.5:
2.6.4:
2.6.3:
2.6.2:
2.6.1:
2.6.0:
2.6.0-test9:
[http://lwn.net/Articles/44243/ libata driver architecture] (Jeff Garzik)
2.6.0-test6:
[http://marc.theaimsgroup.com/?l=linux-kernel&m=104344596912094&w=2 32-bit dev_t] (Andries Brouwer, Al Viro)
2.6.0-test3:
[http://www.nsa.gov/selinux/index.html SELinux] (Stephen Smalley, SELinux team)
2.6.0-test1:
[http://www.linuxvirtualserver.org/ Linux Virtual Server layer] (Wensong Zhang)
2.5.75:
[http://marc.theaimsgroup.com/?l=linux-mm&m=104529418208788&w=2 Anticipatory Scheduler] (Nick Piggin, Andrew Morton)
2.5.71:
[http://marc.theaimsgroup.com/?l=linux-kernel&m=104981044405395&w=2 Switch the IDE I/O layers to taskfile](Bartlomiej Zolnierkiewicz, Alan Cox)
2.5.70:
[http://www.linux-usb.org/gadget USB gadget support] (David Brownell, Stuart Lynne, Greg Kroah-Hartman)
2.5.69:
[http://lwn.net/Articles/29555/ New interrupt handling API] (Linus Torvalds, Andrew Morton, etc.)
2.5.65:
[http://www.kerneltrap.org/node.php?id=603 Desktop Interactivity Improvements] (Linus Torvalds, Ingo Molnar)
2.5.63:
[http://high-res-timers.sourceforge.net/ POSIX timers] (George Anzinger)
2.5.60:
- New modversions implementation (Kai Germaschewski)
- 64-bit jiffies (Tim Schmielau)
2.5.59:
[http://home.arcor.de/efocht/sched/ NUMA aware scheduler extensions] (Erich Focht, Michael Hohnbaum, Martin Bligh)
2.5.57:
[http://www.zipworld.com.au/~akpm/linux/schedlat.html Remove long-held locks for low scheduling latency] (Andrew Morton, etc.)
2.5.54:
[http://secure.netroedge.com/~lm78/ Add drivers for hardware health monitoring] (lm_sensors team)