I'm used to smooth upgrades of both single packages and full distributions, with Debian and Ubuntu. Not so for this upgrade from 9.10 to 10.04. I'll sum up the failures I encountered and my suggested fixes:

• after upgrading+rebooting, your native IPv6 is suddenly replaced by a freenet tunnel. Cause: you installed gw6c on 9.10 which came with no configuration; 10.04 updated the configuration to automatically use freenet. Fix: remove gw6c package.
• you're using the excellent dovecot-postfix package, and after the upgrade, no mail comes in. Cause: cmusieve is gone. Fix: change dovecot config to use sieve.
• lighttpd will not start, complaining about port 80. Cause: lighttpd ipv6 is basically broken in 10.04. Known issue, not fixed. Fix: add server.use-ipv6="enable" to lighttpd.conf
• dovecot is not listening on v6 anymore. Cause: the upgrade messed up your listen directives. Fix: add listen = *, [::] to dovecot.conf

Extra free tip: get rid of some old kernels, they tend to eat a lot of diskspace with their modules.