Yesterday I upgraded my Debian server from Debian 6 (Squeeze) to Debian 7 (Wheezy). Overall it went fairly well, most probably because I don't use that much software. Another reason is that two main packages I use, Nginx and PostgreSQL, were drawn from Squeeze backports so they were close to Wheezy version.
Having the important upgrade notes of all packages at the very beginning of the upgrade was very helpful.
I had nonetheless two big issues with Dovecot and PHP as CGI.
I had to upgrade from Dovecot 1.x to 2.0 configuration file. Dovecot 2 is supposed to be able to read Dovecot 1 configuration file but it did not work for me. First of all, I had to fix the import of the SSL certificates (easily done with help from README.Debian.gz). Secondly, I use non-standard ports and I was not able to easily fix it.
Overall, it was much easier to write a new Dovecot 2 configuration file from scratch. Using
doveconf -c -n (also mentioned in README.Debian.gz) was very helpful to get the items to modify/add.
I don't see what Debian developers could have done better, the issue was at least well documented.
PHP as CGI
I am using Nginx web server so I had a custom init.d script to launch PHP as Fast CGI, Nginx and PHP communicating through a Unix socket. I don't know why but my PHP as CGI set-up was broken after the upgrade.
I easily fixed this issue by installing php5-fpm package and using the proper socket (
/var/run/php5-fpm.sock) for the Nginx to PHP link. My server configuration is thus more standard and easy to maintain. Good! :-)
Feature wish for Debian 8
For next Debian, it would be useful to have a script that scans the installed packages and prints some notes telling if the upgrade can be done automatically or need manual intervention (and why, pointing to some further documentation to read). It would be very useful to know issues before starting the upgrade.