Coming from Kubuntu LTS, here are my reasons:
I've broken Kubuntu simply by updating it in the past, and LTS software availability becomes obsolete by the end of it's life cycle, so I knew that I want a rolling release setup is ultimately what I want.
By being on Debian "Testing," I'm essentially on a rolling release platform. I don't have to reinstall my OS in 6 months, 1 year, 2 years nor 3 years. I did the installation once, and I can expect to have recent software without the nonsense of reinstalling a new version all the time.
The competition in this regard is Gentoo, Sabayon, Arch, Chakra, and Manjaro:
* Both Gentoo and Sabayon have broken on me when updating, and I've tried these Distros more than once in the last couple of years, and I kept breaking it, so never again for me.
* I didn't try Arch because quite frankly, after Gentoo, I don't ever want to mess with a command line based installer for days to only decide that it's not for me.
* I have tried both Chakra and Manjaro recently, and they both update without a hitch and are easy to use, but I didn't like Chakra due to it's limited software availability, and Manjaro is basically bloatware central.
For Debian, I used the 7.0 Live DVD KDE edition, and the installer wasn't hard per say, but it wasn't as straight forward or as clean as Fedora's installer or even Ubuntu's. I had to get my hands a little bit dirtier with getting my wireless card to work since they're non-free, but I figured it out which driver to install from the non-free repository. After that, I switched my sources.list to "testing," and everything was clear sailing after that. So I have Debian with a basic KDE setup, and I've customized it to exactly what I wanted without extra bloat. I event found a Debian KDE start logo online (for some reason, Debian forgot to include it).
Debian solution is the most supported and it simply works. It's the most complete out of all the Distros, so it's an automatic win for me.
My setup gets tested on "Sid Unstable" before it comes my way to "Jessie Testing," so it's a happy medium in the stability vs recent software debate.
I feel that this is a real community without the drama that happens in the other Distros. I've read the forums here, and I feel people are definitely genuine around here.
*Ubuntu - I vote no on Unity and Amazon Spyware on by default. Even though Kubuntu doesn't spy (yet?), I don't even want to know what other fast ones Canonical will try to pull on me.
*Gentoo - lots of bickering and thus Funtoo is born.
*Arch/Manjaro - Animosity between the two Distros.
*Mint - I'm not even Jewish, but I took the dev's advice anyways
I’m only going to ask for one thing here. If you do not agree I kindly ask you not to use Linux Mint and not to donate money to it.
That's pretty much everything I can think of at the moment.