I have read this fascinating thread as one who is faced with the need to upgrade because everything wants glibc-2.2. I have used Mandrake for about 2 years, faithfully buying a new box every so often. I feel that I have learned a fair amount about linux with Mandrake, and because I made an effort to figure out how things worked. I think it is a real advantage to have a system that provides rpm's, or another packaging system, so a reasonably good and useful installation can be made, but you have to probe deeply to have an understanding of how things actually work, what is truly useful, what is reliable, and how to avoid problems. I still need to learn how to use programs like databases, apache, ssh, and such, but I just don't have the time. Really valuable knowledge is in learning how to make the system work for you. Compiling programs is fine, but I don't see it as anything special. My real concern with my distribution has emerged as having a system that is reliable and easy to maintain. This, I think, is what you will learn to create with whatever distribution you choose. If you don't, you will choose again.
At this point, I have obtained debian cd's, but I really don't want to go through a tedious install. I will not consider gentoo for that reason. This leaves Mandrake, Red Hat, and Libranet as the candidates. Now, I have learned about installs and how upgrades don't work and how to be very concerned about partitions. I know the gist of Mandrake. I would use Red Hat if I was interested in a real computer job (and probably learn perl, python, php "lamp"). So, really, can one learn anything with Mandrake 9.1, or should I use my distro budget on Libranet?