Gentoo Linux is not for everybody, seriously. And by that I don't mean only the average computer user, but also the majority of application developers. Of course, this is just my opinion, which I will explain below. I do not in any way mean that Gentoo Linux is inferior to other distros either; it's great for people who care about customisation and speed, but not for those who just want a stable, working development environment.
To many people, the biggest attraction of Gentoo Linux is that you are effectively rolling your own flavour of Linux when you install it because you pick, configure, and build everything, even the installation CD. This aspect is different from most other Linux distros for which prebuilt installation CD images and binary software packages/repositories are available. The primary reason for picking, configuring, and building everything from source is the performance gain that results when the compiler and linker are optimising code for a particular architecture, even more so if object code is statically linked.
The majority of application developers, however, do not care about the performance of the OS hosting the development environment. Therefore, the performance gain from compiling everything from source is largely irrelevant to the application developer.
Furthermore, the difference in performance between Gentoo and other distros shall be significantly reduced nowadays. Gentoo, when it was still known as Enoch, used a fork of the GCC that provided about 10% performance gain over the mainstream, official version of GCC. This difference in performance no longer exists since the fork has been merged back into the official version of GCC. If you don't care about the performance difference that remains, you might as well download binary packages; but if you are using binary packages, what's the point in using Gentoo?
The Portage package system that Gentoo uses is also not something that the average application developer would like. Many of the packages have too many build options, and sometimes these very same options are under documented. For instance, the last time I counted, the package for the Apache server has around 60 build options, which do not exactly correspond to the options used with the configure script of the official source package from httpd.apache.org. Documentation of the options is impossible to find; even the popular Gentoo-Portage website has no information (https://web.archive.org/web/20080824031457/http://gentoo-portage.com/www-servers/apache/USE#ptabs). Some of these options conflict with each other; the only way to eliminate all of the conflicting options is by repeatedly trying to install the package, allowing the portage commandline interface, emerge, to report conflicts one at a time, so that you can eliminate one of the conflicting options one at a time.
The application developer has enough to keep his hands busy and his mind on the brink of insanity. He cannot afford to lose time with compiling the development environment.
Gentoo may be great for top notch performance in the live, production environment; but if anyone asks me to pick a distro for development environment, I would pick Ubuntu over Gentoo any day.