This article is interesting although it probably doesn't introduce a whole lot new (i.e. it's been said before here?):
The problem I see from Ubuntu, although I'm not all that familiar with the entire issue, is that Ubuntu seems to do a lot differently which can have a somewhat negative impact. The more they 'deviate' the more it's harder to have everything flow, imho. There's what, 100 distros or whatever and it's one of the situations you get. You have to take the good with the bad and admit the disadvantages as well as the advantages.
Ubuntu forked Debian but a lot of debian-based distros stay relatively aligned so that it's mostly just a lot of tweaking. I don't think Ubuntu just 'tweaked' Debian. Also, the sudo/su thing. They seem to change things drastically and they have the resources to do it and market their products. There also seems to be an overwhelming priority of resources on Gnome whereas some other distros seem to achieve a balance even if they have a certain default DE. Kubuntu is perceived as a poor man's KDE distro although other distros do well with the KDE integration or development. The other distros doesn't even have the same resources but their KDE ver. is said to be solid (Arch, maybe? Debian, itself?). Why is that?
I'm neutral on Ubuntu but I concede the complaints/concerns and I suspect they may be valid. But, I do use it because it's popular so the primary reason is on a practical basis. It might not be the best reason but I figure I can't go wrong. Also, the other major reason is the connection to Debian and Debian is my main distro so...
Carry on... I thought the article might be of interest and it's relatively up to date (as of this post).