Differences between 2.4 and 2.6 were quite big, I don't think there was another such big change in kernel releases any time later. But that was also the time when Linux was transitioning from being a hobby project (already useful for serious stuff) to being a serious professional operating system – the last moment for major refactoring.
Linux kernel is still changing and being constantly refactored, but now the changes tend to be more gradual and version numbers matter much less.
I have never used Arch Linux, but have read their wiki a lot. It is really very useful.