Live migrating Ubuntu to Debian - introduction

I was a long time Debian user before I switched to Ubuntu in 2005. I always liked Debian for its stability, wide choice of software and superior package management. However, at this point Debian release cycle, always long and unpredictable, got even longer than usual. A stable or even testing version of Debian meant seriously outdated software, not to mention poor hardware support. With Ubuntu, my laptop finally got a decent power management out of the box. There were other improvements, like NetworkManager and a general focus on ease of use. Plus, being a Debian derivative, Ubuntu inherited many good traits of its ancestor. It was a good choice than and for a few following years.
Then something began to break. There were some controversial design choices, but even worse was declining quality. For the last few releases I had to deal with numerous bugs, some serious, most minor but annoying. 9.10 was pretty bad, but a last normal release before LTS is always a playground for new technology. But 10.4 is even worse and it's supposed to be a Long Term Support release! Everyday I install a few updates (yes, I'm happy the bugs are getting fixed, but there shouldn't be that many in the first place) and yet my machine is still unstable. My box would sometimes lock up or get a load average of 15 for no appearent reason.
In the meantime, Debian release cycle got shorter and predictable. Many Ubuntu usability improvements were merged back upstream. Some widely reported features are not useful at all for me: I don't need Ubuntu One, I use unison on my own server; I don't need Software Center cause I prefer Aptitude etc. I decided it's time to move back. Clean install would be an obvious choice, but I decided to migrate a live system instead. There were some issues, but it worked. Did I mention that Debian has a great package manager?
If you want to know how to migrate from Ubuntu do Debian, stay tuned for the next part (update: it's here). In the meantime, you can read my post on downgrading Ubuntu. If it seems complicated or dangerous, be warned that migration to Debian is quite harder.