Why is it bad to run as root?

I've often come across posts on forums or other websites where you see people joking in such a manner about running/logging in as root as if it's something awful and everyone ought to know about it. However, there isn't much that a search reveals on the matter. It maybe widely known to Linux experts, but I really don't know why. I remember always running as root when I first tried Linux years ago (Redhat and Mandrake) and don't remember running into any problems because of that. There are actually some distros that have a bright red background with alert signs all over it as wallpaper for the root user (Suse?). I still use the "Administrator" account for regular use on my Windows installation and haven't ever run into any problems either.

These users have administrative access to the system, a permission level that allows them, and applications they run, to circumvent protection mechanisms (such as file system permissions), allowing them to do everything on a system. Malicious software loves unrestricted access.

As a simple example,

sudo rm -rf *

will delete pretty much everything on a Linux system, if you're in the root directory. It needn't be as obvious as that, or even malicious. Something like unexpectedly undefined variables in scripts that require root privileges can cause severe problems:

rm -rf $destinationDir/*

# if the variable is undefined and the shell doesn't care, the result is:
rm -rf /*

Sometimes, it's a simple typo in a script that deletes half your system.

Therefore, the general recommendation is to use execute programs only with elevated permissions if it's really necessary.

Root on linux is even more powerful than an Administrator account on Windows. It is comparable to the SYSTEM account on windows, which is normally not accessible.