What Defines a Linux Distribution
A Linux distribution is more than just the look and feel of the desktop. A lot of different things go into making a distro what it is, and you should try to take them all into account as you search for the right one. While most of these are customizable in any Linux distro—that is, if you don't like what apps, desktop environments, or drivers come pre-packaged, you can always install them separately—the idea is to find the distro that's as close to your ideal setup as possible. That way, you spend less time fiddling and getting everything the way you want. Here are some big things you'll want to keep in mind.
Package Managers
Traditionally, one of the biggest things that separates Linux distributions is the package manager. Those of you that have used Ubuntu are probably familiar with APT (or its graphical interface, Synaptic), and you'll find that same package management on Mint, Debian, and other distributions. However, other distros have their own package managers. Fedora's Yum manager, for example, is a tad easier to use from the command line than APT (though it can be a bit slower at times).
More important than ease of use, though, is the availability of packages. Since Fedora is not quite as popular as Ubuntu, Mint, and other Debian-based distros, it may sometimes be harder to find the app you're looking for—which means you'll be stuck building from source instead of just installing it from the repositories. Building from source isn't all that hard, but it won't allow you to automatically update that program when a new version comes out, which is a real bummer. So, consider both the ease of use and the popularity of a given package manager in your choice of distro.
Desktop Environment
The basic look and feel if your windows, desktop, and menus, as well as how customizable it is
Resource usage
The options available in your distro's graphical preferences (such as remapping certain keys, changing what items appear in your menus, and so on).
How well certain programs integrate with your desktop and each other
While you can always install a different desktop environment, it's probably still worth considering heavily. The closer the default desktop is to the desktop you want, the less work you'll have to do to set everything up, and the better all your programs will work well together.
Stability vs. Cutting Edge
Different distros have different release and update cycles. Some, like Fedora, focus on having the most up to date versions of all your apps and packages, so you always have the latest and greatest. However, this can sometimes come at the expense of stability, which is why other distros—like Debian—prefer to delay certain updates to make sure everything runs smoothly. If you always have to have the latest version of Firefox or any other app, something like Debian probably isn't right for you—you'd want to choose something like Fedora which will be a bit quicker to get those updates out to you.
Hardware Compatibility
Different distros package different drivers in their installers, which means that, depending on which distro you use, you'll find that different pieces of hardware may or may not work out of the box. While you can often get other drivers installed with a bit of extra work, it sure isn't fun. As you're looking through distros, check their hardware compatibility pages (or test them out with their Live CDs) to see whether things like your Wi-Fi card, video card, and sound are all compatible out of the box. If not, just know that you'll have to do a bit more work to get everything up and running when you first install.
Community Support
A big part of Linux is the community surrounding it, whether for troubleshooting, app support, or even good documentation. The larger the community surrounding a distro, the more likely you are to be able to get help, find documentation on a specific problem or piece of hardware, and otherwise get information. This is what makes Ubuntu such a great beginner distro. Poke around the forums of your chosen distros, and see which ones fit you well.
Distributions Every Linux User Should Try
Now that you know what makes a Linux distribution, you may be wondering where to start looking. DistroWatch is an incredible resource for those looking to branch out, but again, there are hundreds of distros out there, and it can be pretty overwhelming. While I highly recommend exploring beyond our paltry list below, here are a few distributions that are incredibly popular, and are great starting points for any search. Note that most distributions have variations that use different desktop environments, but we'll focus on the default environments for each here.
The Standard: Ubuntu
For the Beginner: Linux Mint
For the Bleeding Edge User: Fedora
For the Cautious and Stable: Debian
For the Tinkerer: OpenSUSE
For the Diehard: Arch Linux
Again, check out DistroWatch for more ideas, and don't be afriad to test a number of distributions before you settle on one—the more you test, the more likely you are to find the one that fits you perfectly.
Got your own favorite distro, or any other comments on how to pick the perfect one for you? Share them in the comments.