Believe it or not GNU/Linux distributions are not supposed to be 100% working out of the box, nor is any OS, so I don't really understand the "incomplete" reference there. ubuntu has pretty large repositories, so it's just a case of finding what you want and installing it.
Derivative distros are just customised distros where someone else has done the work for you, usually by selecting a particular DE and doing some preconfiguration and installing some extra (usually proprietary) software. Many of these derivatives we're seeing recently are based on ubuntu, which is itself a derivative.
This of course makes it "easy" and the project team behind the OS will come up with some catchy marketing to reflect this. It's always "easy" until something breaks - then the derivative users are clogging up the Debian or ubuntu forums asking for help because the derivative forum isn't active enough and because their derivative is "the same" anyway... "if I had a dollar for every time I've seen that one"... well you get the picture...
If you want to learn about how to set up, configure and administer any *nix system then you will have to do some things for yourself - this is even going to be the case with Debian derivatives and with distros like openSUSE and fedora. Even when you can do 90% via the gui, there will always be that 10% which will require the use of a terminal - this is seen as a failing of GNU/Linux when in fact it isn't. There is no GUI in existence which can fully replace the CLI on a *nix system and no serious, read 'sane'. developers are interested in producing one. This is because a) the CLI is the best and most powerful tool available and b) not all GNU/Linux boxes run X after all.
The problem is of course that some people only recognise progress or advances if it's appearing on their monitor in high definition 3D with translucency and animated flashy spinny stuff...
There is always a learning curve...
If you take windows for example, in a few years time, installing windows 7/8 will be much the same as installing windows xp is today - a messy procedure involving downloading and installing tons of drivers, disabling useless services and features and downloading a gazillion updates. Many people judge GNU/Linux distros quite harshly in terms of ease of use and ease of install, but forget that they had to learn to install and set up windows - and that took years. Others I see criticising .e.g the installer, have never installed their windows OS in the first place.
GNU/Linux was originally an OS for hackers by hackers and that's how it will probably remain despite the best efforts of some corporate entities to dumb it down in order to produce a product which can turn a profit. I see understanding the OS and making informed choices as a good thing.