Another possible interpretation of user-friendliness is Ubuntu's thoroughness in building a commercial brand. Its family of subjects -- Kbuntu for KDE, Xbuntu for Xfce, Edubuntu for education -- has the advantage of assuring users that, if they want something beyond the basic GNOME desktop of Ubuntu, their experience will be much the same, no matter which they choose.
Other features, such as the third party repository for applications, the ShipIt service for those who want a CD rather than a download, and the availability of pre-loads from Dell or retail software boxes from Best Buy may all be primarily due to commercial considerations. After all, Canonical, the commercial arm of Ubuntu, is not profitable and is unlikely to be so for some years.
However, these features are also user-friendly in the sense that they give an experience not much different from what they know from proprietary applications. When dealing with Ubuntu, users are in many ways on familiar and comforting ground. At the same time, more experienced GNU/Linux users are unlikely to be offended, because, while Ubuntu has overtones of a commercial company, it still manages to be an inoffensive FOSS citizen.
From several angles, then, Ubuntu deserves its reputation for user-friendliness -- even if it needs to be qualified. Still, user-friendliness can sometimes come at a cost.
In Ubuntu's case, one of the costs may be a gradual slowing of innovation. In part, any such slowing is an unavoidable consequence of mature software. At a certain point, thinking of something new becomes much harder simply because there are fewer immediate improvements. But in recent years, the innovations seem to have become fewer, with Fedora and openSUSE introducing new features like PulseAudo or PackageKit before Ubuntu does.
Another possible problem is quality control, if grassroots commentary is any indication. The gist is that Ubuntu is not checking its packages with the same thoroughness that Debian does, and is slow to respond to bug reports. Instead, Ubuntu's concern seems to be having the latest version numbers. However, since hard figures about these concerns are not available, the problem may have more to do with rumor than any reality.
Yet another cost may be a drift away from concepts of software freedom in favor of practicality. While Mark Shuttleworth, Ubuntu's benevolent dictator, has an interest in software freedom, to judge by the number of times it is mentioned in his blog, in practice, whenever Ubuntu has a choice between software freedom and usability -- or, perhaps, commercialization -- software freedom seems to lose out.
The most obvious example of this tendency is Ubuntu's Hardware Drivers application, which helps users to download proprietary drivers. You might argue that the application simply acknowledges the inevitable, since many people want the proprietary drivers, and will install them anyway. If Ubuntu did not provide them, a less reliable sub-culture dedicated to providing them would simply emerge, as has happened with Debian and Fedora. All of which is true -- but Ubuntu might just as easily have devoted some of the energies put into this application to helping projects that are developing free software drivers, so a choice is clearly being made.
Ubuntu isn't the first distribution to struggle with usability and its relation to other issues. However, from most angles, it does seem the distribution that has spent the most time dealing with such things -- so, yes, the epithet of "most user-friendly distribution" does seem to apply.
However, answering this question is not just an end in itself. Without judging Ubuntu's decisions in any way, by looking at how it has navigated the issues, you get a snapshot of the current state of GNU/Linux distributions, especially those caught between traditional community values and the needs of commercialization.
To many, Ubuntu's choices seem sensible and inevitable. But to others, especially those familiar with old school GNU/Linux, they also raise the questions: Is user-friendliness all that a distribution needs? Or are there other values equally important?