Coming from Kubuntu LTS, here are my reasons:
Rolling Release -
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).
Package Management-
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.
Stability -
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.
Community -
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.