Debian – Automatic security updates on Debian squeeze server

About your requirements, Iceweasel is the supported Firefox derivative (fork), I'm currently running debian as my desktop OS at work and use iceweasel every day, no problem. Gnome3, I think it'll be available on the next stable release, BTW what release are you running? Squeeze? If so I think (IIRC) Wheezy will have it. And finally, about graphics performance/quality, that depends a lot on your graphics card and its driver, but if you think of it like having transparencies, windows closing with fancy effects and so on, you'll need a moderm desktop or compiz (work with gnome2) which I think its available on stable (wheeze).

I have modified a bit your sources.list for wheeze, do you mind to test it and report back?

deb http://http.us.debian.org/debian/ squeeze contrib non-free main
deb-src http://http.us.debian.org/debian/ squeeze main

deb http://security.debian.org/ squeeze/updates main non-free contrib
deb-src http://http.us.debian.org/debian/ squeeze/updates main

# squeeze-updates, previously known as 'volatile'
deb http://mirror.cse.iitk.ac.in/debian/ squeeze-updates main non-free contrib
deb-src http://mirror.cse.iitk.ac.in/debian/ squeeze-updates main non-free contrib

# 3rd party repositories
deb http://packages.dotdeb.org squeeze all
deb-src http://packages.dotdeb.org squeeze all
deb http://www.deb-multimedia.org squeeze main non-free

If you happen to be using wheezy or sid you'll have to change all squeeze ocurrences for the one you're using.

Please, backup your sources.list before replacing it, then test it as follows:

Refresh caches

# apt-get update

Search package

# apt-cache vlc

Install package

# apt-get install <package_name>

If find trouble, please report back with output for those three commands, or at least for the first and last.

Also, if you are already using Wheezy or sid, forget what I said about Gnome3 not being available, it should be there but wheezy is yet to be release and sid is always the development branch.

I moved from Ubuntu to Debian a couple of years ago and never regret this decision. Concerning your questions:

1. You can use different branches of Debian. As a new user I would recommend the stable branch (which is indeed very stable but sometimes lacks new software) or the testing branch (which is only a little less stable but provides newer software). Both branches provide security updates. They are installed every time you do a system update.
2. Debian Stable doesn't get any new software - only bug fixes and security updates. Debian Testing is rolling release *), meaning that new software is provided continuously. This is a difference to Ubuntu, where you have to upgrade to the new version every once in a while to get new software.
3. I cannot answer this question as I have never used Cent OS. I heard that it is a good, stable distribution. As it is used for servers, it also should be quite secure.

Coming from Ubuntu, you might want to consider that Debian is more similar to Ubuntu (to be precise, Ubuntu is built "upon" Debian). Both, Debian and Ubuntu use Apt. Cent OS is a derivate of Red Hat Linux and uses RPM instead. There is nothing wrong with either of them, however you might already be more used to the Debian approach.

*) To be precise, just before the current testing release becomes the new stable release, there is a so called "freeze". In this time window testing doesn't get any new software - just bug fixes. After that, when the new stable release is out you have to perform a dist-upgrade (apt-get dist-upgrade) to update your system to the new testing-release (if you want to do so, make sure your /etc/apt/sources.list contains the word testing instead of the name of the current testing release, e.g. stretch).