I'll assume Ubuntu as your distribution as people seem to be picking this up and calling it "Linux", there are many distributions, all with different goals.
Linux does, can and will happily play DVD movies but it does not come with many codecs at all which allows you to choose your own. I personally have a lot of codecs in my Ubuntu systems as they are media PCs (movie / music playback). However someone developing a system will not require these as it is not the intended use of the PC. You may have noticed that an MP3 codec is not included butmay seem standard if you come from Windows world.
In Linux you have choice and options so very little is given to you but it is extremely simple to install what you need (especially in Ubuntu where the repositorys where you have been updating and installing free software form).
Anyhoo now you know WHY the codecs are not bundled in. Lets get you sorted
NEVER EVER sudo to run graphical apps as root (like kate or gedit). Ignorant idiots will tell you this is right. IT IS NOT.
Sudo is for command line programs like cp, mv, apt-get, ln etc etc. If you want to edit a text file with gedit as root use gksudo gedit <file> (or for kde its kdesu kate <filename>)
if you sudo to GUI apps, the environment is not configured properly and permissions and file ownership can be modified and damaged to such and extent that the files will only be accessible by root and you will not be able to bootup without having to change permissions on the faulty files first
YOU HAVE BEEN WARNED
Not a personal attack. Just that you sound new to Linux and I dont want you breaking your system by following incorrect instructions you may find online
It was ubuntu but I have tried red hat and edubuntu before. I think there need to be a "multimedia distro" which will run dvds without any additional meesing around like the way openoffice is installed. Just a thought.
I use Centos 4 on my servers and my first linux distro was a great product called e-smith now contribs.org.
You will always have some element of "messing around" as the distribution you choose will not fit your needs as the stock install attempts to cater for the masses.
Maybe not everyone has a DVD player or wishes to play DVDs? Maybe they dont want burning software or a chat client? The distribution team make these choices of what is standard when the image is created. The fact that you have to install MP3 codecs due to legal reasons is also "messing about" but im pretty sure you installed it. This is the same with DVD playback, the codec is not installed but there are thousands of walkthroughs on how to do it.
Dont be afraid of a little work in setting your system up, onnce its done, its done and you can get on with enjoying your creation.
If you need any Linux advice then werd me up on here or email me. Im usually here etc.