Fedora is the "free brother" of Red Hat. Things that work in Fedora (new, bleeding edge features) trickle upstream to Red Hat Enterprise Linux, which is an enterprise business variant of Linux. Recently, they just broke a billion dollars in revenue (eat that, closed source zealots that swear you can't make money on open source). Fedora comes with Gnome Shell, as they've traditionally been a Gnome based distro for the most part. Some of it may be because a lot of Fedora/RHEL developers are also Gnome developers as well. Ubuntu is very Gnome entrenched, but they of course want to do their own thing. While Unity appears to be a "new desktop environment", it's built directly on top of Gnome. My hat goes off to them for betting against the odds and in one short year later updating Unity to be, not only somewhat usable, but rather kick ass. That said, I still have a ton of respect for Gnome Shell and still frequently use it.
Fedora is more of an open source zealot, though, which would make Richard Stallman quite happy. For example, "back in the day" when Broadcom sucked with driver handling (closed source drivers, probably the most ridiculous thing I ever heard of), it was more of a bear to get Fedora working with Broadcom. It usually required plugging in via ethernet and updating, then downloading the driver and uncompressing it, etc. Lately I heard more talk from my cousin (big Fedora fan) about enabling RPM Fusion, which is an additional repo in Fedora that brings down a lot of proprietary software. Seems easy as pie, right? Well, it is, but it's not every day you realize your Grandma can pick up on what a repo is and how to enable it, despite the fact it's just a few steps.
The difference is, Ubuntu itself is open source, but comes packages with *some* closed source software, which is why it's easier to install drivers and whatnot that are still closed source. Fedora just requires you to download the repo before you can branch out further.
In my experience, Fedora, despite being pretty bleeding edge, is pretty rock solid. I'm not sure I'd use Fedora in a core server environment, mostly because there's CentOS, the free fork of RHEL, but in my experience it's been decent to use. Personal opinion, though, is I'm not the biggest RPM fan. I've been more of a Debian guy. That said, that's another +1 that keeps drawing me back to Ubuntu. Easy to use, easy to configure, Debian based? I'm all right with that.
For what it's worth - Gnome Shell should be easily installable in Ubuntu, Mint, etc. I used to run it in Ubuntu heavily.
Quick reference:
RHEL = Very stable, open source, but costs money because it comes packaged with support out of the box.
Fedora = More of a "guinea pig" (but a pretty awesome guinea pig) for RHEL.
CentOS = Not affiliated with the RHEL/Fedora relationship, but directly based on the RHEL code. Open source with no support costs. It's basically RHEL with no official support (besides forums, Google, etc.) and no cost.
Ubuntu = Debian based distro. Free, but additional support available through Canonical. I find Ubuntu to be a bit less "bleeding edge" than Fedora, while allocating some focus at stability (hence LTS releases, etc.)
Mint = Initially based on Ubuntu, but also has a Debian edition. It's basically Ubuntu + codecs (which is a simple checkbox these days in the Ubuntu installer). Personal opinion: Mint is *that* similar, I wonder why it's even forked... It's basically Ubuntu with a checkbox already enabled to install codecs + a different interface that's installable in Ubuntu.
Mageia = Based on Mandriva. Mandriva has been riddled with bankrupcy issues for years. Quite sad, because a Mandriva dev is actually responsible for creating K3B, a really awesome burning application. When you know what hit the fan again, a lot of developers bounced to create Mageia, which is based on Mandriva but with a different management path in mind.
Quick notes about Mint: Mint comes with certain multimedia codecs by default, which in the grand scheme of things, I understand is somewhat illegal. Evidently the codecs need to be initialized by the user to install in order for it to be legal, because that proves that the user initiated the download and the distro didn't come pre-packaged with it, which would have held them responsible for certain royalty charges. In the grand scheme of things, Mint is very small, so nobody will give a damn to bark about it. That said, it's some food for thought if you want to use Mint in a business environment, because that little checkbox Ubuntu provides evidently gets them off the hook because the checkbox is user initiated.
