In mid January 2001 I and my brother Björn Stenberg (independently) upgraded our kernels to the 2.4.0 series. I decided to collect our experiences here...

Lots of things. Joe Pranevich wrote an article about what's new in 2.4.

A few highlights include: better internal effieciency, no more 1024 processes limit, bigger memory support, NFS v3 support, ISA PnP, USB support, I2O support, PCMCIA support, rewritten network layer, big files support (>2GB), improved SMP....

• If you're using PPP you need to update to pppd 2.4.X.
• The kernel module stuff is modified, so you probably want the new modutils 2.4 as well.
• If you're doing any 'ipchains' magic you might want to enable "ipchains (2.2-style) support" in the kernel config. The 2.4-style is to use iptables and netfilter instead.
• If you run one of those crazy redhat 7-installations, you need to edit /usr/src/linux/Makefile to use kgcc when you compile the kernel.

Otherwise, nothing was much different. It was much the same as upgrading kernels usually are.

Tigran Aivazian's in-depth article about the 2.4 Linux Kernel Internals is very interesting!