I'll say it unambiguously one more time in case it wasn't absolutely clear from my original report: a default build with no additional configure options specified must either result in a build where basic applications (Qt designer, assistant, demos) are usable or result in configure terminating with an intelligible error message describing why a useful build cannot be provided.
Many people I talk to around the office have fallen into the same trap with missing xcb packages and have each wasted some hours to get to the point of having a usable build. Clearly the current behaviour is not helpful for our own development staff, let alone for the user community, where knowledge of packaging systems and Qt's dependencies on them is possibly more sparse.
Regardless of spurious arguments about technical purity, the simple fact is that almost every user who has a system that builds Qt4 without issue cannot do the same out of the box with Qt 5. Some of these people will be fortunate to find someone who knows the solutions, some will not and will probably give up on Qt5, for a while at least.
Sure, in late 2013 when all distro's have migrated from Qt4 to Qt5 and thus have the xcb packages installed, this won't be a common problem. In the meantime however, we are clearly doing a significant dis-service to both Qt contributors and those Qt4 users who are just attempting to try out Qt5 for the first time. This is not good customer service.
At a minimum, point 4 from Lincoln's comment (two posts up) should be implemented. Ideally, there should also be documentation updates to the What's New in Qt5 and to the pages that describe building on unix to tell users about the new scheme and what packages they should install to get a usable Qt. this information should also be posted on Qt Labs and Qt5 mailing lists as a matter of good customer service for our community and early adopters.