Recently I have heard several people remark that Ubuntu is not a "real" Linux distribution. It's not surprising that they got that idea, given the amount of criticism that Ubuntu gets in a typical week. They think that being beginner oriented means that it has to be dumbed down.
Ubuntu was my introduction to Linux. I used it as my primary OS for eight months. During that time I dual-booted it with a dozen other distros and used countless others in virtualbox and my junk computers. Then my filesystem got corrupted and I decided not to reinstall it. Now I am happily using Arch and Debian testing while still keeping Ubuntu up to date in virtualbox.
Anyway, I can tell you that Ubuntu is very much a traditional Linux distribution. You can get to the command line with a simple ctrl+alt+f1-f6, updating is a simple sudo apt-get update && sudo apt-get upgrade, and you can use any window manager you please with the xinit command or from your display manager. You also benefit greatly from the Debian repository.
So what are the differences? It is easier to install drivers and proprietary plugins and codecs, there are lots of visual enhancements like putting the music player controls in the volume menu and it uses newer software that in some cases is not fully tested.
There are also a lot of changes in store. Unity will replace Gnome in 11.04 and Wayland will replace Xorg sometime in the future. There's no point in worrying about Unity since it's just a window manager, but Wayland will require applications to be rewritten and needs a faster video card. However, I suspect that X will be still provided as an optional package and it may be possible to have both installed at once and start whichever one you want.
My feeling is this: every distro has the right to do things their way. That's what Linux is all about. Proprietary codecs are a must for non-technical users. As for the software, you always have to balance being stable with being up to date. You can't have both and where the happy medium is depends upon the usage. There are things I don't like about Ubuntu. And as it is often seen as the face of Linux, and problem with it reflects upon Linux as a whole. So if you don't think it is suitable, don't recommend it. If someone complains about an Ubuntu-only issue, refer them to a different distro. But if someone uses it and is happy with it, don't try to tell them that they shouldn't be or that it is in some way inadequate. Ubuntu is happy to give it's users complete control over their environment and they can learn quite a bit if they are so inclined.