I think it's fair to consider them as different distros, at least in practice, given how much the choice of a desktop environment can affect the user experience. A lot of the software available by default will differ, obviously. Instructions about how to use the system differ, as well. The default capabilities and functionality will differ. The bugs affecting them will differ. All of those differences add up.
"Different defaults" is not really something that differenciates distros. Lubuntu, Xubuntu, Kubuntu etc have a name: They're remixes of an existing distro.
That's not very good reasoning. You're basically saying that Fedora and Ubuntu are the same distro, because differences in the default package manager and other core software are irrelevant. Clearly that's not the case.
It is the defaults that matter. They're what give a particular distribution its unique characteristics.
Sure, you can go out of your way to install the software necessary to make one distribution comparable to some other distribution, but that doesn't mean they're really the same to begin with, even if their names may be similar.
What makes distributions different are mainly the package manager and the packages in the repositories.
Your example with Fedora and Ubuntu is a bad one because they use different package managers. Fedora uses yum and Ubuntu uses apt. This is one of the biggest differences that two distributions can have. They are different enough people get advice to stick to one and build some experience with it. If a person is using Debian on the servers, then he better stick to Ubuntu or Mint as a desktop (if he uses Linux as a desktop at all). On the other hand CentOS users are better off with Fedora or OpenSUSE.
The difference between Debian, Ubuntu and Mint is more subtle but they are still different. For one they are maintained by different people. One could argue that Xubuntu and Ubuntu are also maintained by different people which is true for the Desktop Environments, but it is also true that the kernel of both is maintained by the same people and they use the same kernel at any given time.
Another one is that Ubuntu and Xubuntu have exactly the same repositories for a given release. Mint is based on the repositories of Ubuntu and many (most) packages are the exactly the same, but then Mint has some extra packages specific to their distro.
As a result most of the time you can follow a 14.04 guide and apply it to Mint 17 and everything will be OK but there is the occasional difference.
"What makes distributions different are mainly the package manager and the packages in the repositories."
...and the packaging/updating philosophy, the packaging/defaults, the configurability. How they handle your hardware (hotplugging printers, monitors, mice). Batteries included or not. Power management.
E.g. Fedora always uses the latest upstream packages and doesn't patch them very much. Debian/Ubuntu/Mint use stable packages and patch them heavily. Many years ago one such patch in the (stable) Ubuntu kernel broke my (officially supported) file system, causing data loss. OTOH, Fedora can be a bit harsh to use for newbies.
