1. Debian is much more cool. It is a co-operative. People are motivated by doing excellent things.
2. That motivation should be nurtured. Many Debian contributors like to contribute to a product called Debian. Ubuntu relies on Debian but disguises this. I didn't feel good about it, as someone who has been a volunteer and community contributor over the years.
3. The Ubuntu release cycle results in pretty severe compromises. Debian has "testing" in which a continuous stream of quite mature packages are introduced. The bulk of the effort of the Debian and Linux developer community ends up in testing quite soon, but nothing goes into testing without some time in an oak barrel.
Every six months, Ubuntu takes a snapshot of Debian "unstable", spends a maybe three months trying to get this working well, sends out a release, and then does it again. There is not much capacity left to update the last snapshot, so if something is broken you basically have to wait until the next release. It's like the Debian cycle, except compressed greatly. Feisty had some problems that really made it clear to me that the Ubuntu model may not be reliable enough for me. I don't mind problems, but the fact that they are not fixed annoys me.
I think the reputation that Debian is very conservative and Ubuntu is more cutting edge is seriously misinformed. I think the Debian release cycle, which is the result of a great deal of experience, is superb. You can choose your mix of agressiveness vs stabilty. Testing is an excellent compromise.
Ubuntu paid Debian a great compliment. The money behind Ubuntu could have more directly changed Debian: instead there is a healthy cultural firebreak. Canonical could have used another distribution, but I really believe that the community spirit of Debian was greatly admired. Reading about developers who have left Debian to focus on Ubuntu, it seems clear that the more centralised leadership model of Ubuntu could be a good benchmark for Debian. Also, I think the Ubuntu community support is better. Finding help for Debian is a bit like navigating a old harbor at midnight on a night with no moon. You have to steer around a lot of web pages and howtos which are old hulks that should really be sent to the bottom of the ocean. So I have to nominate this as an area I cancontribute to.
I'll update this after some more experience, but after two weeks on two machines, the experience so far is excellent. Ubuntu was on balance very good: it was Ubuntu that gave me the confidence to move away from Windows (the packages for vmware server and for Nvidia made it so convincing, and I was amazed that suspend and hibernate finally worked under Linux)