Pros:

01# Old systems-Works well even on older systems without compromising on features.
02# Visuals-It has all 3D and other visually pleasing effects. Some of these effects surpass those of Vista (like desktop cube.)
03# Stability-It is very stable and never crashes. Also, it is the fastest.
04# Safety-Being built on the strong foundation of Debian linux, it has loads of safety features, many of which you won't find in other systems. As you guessed, you won't require to install any anti-virus softwares.
05# User-friendliness-It is the most user friendly linux distro around.
06# Driver availability-Drivers for most devices are available for linux, and when you connect to the internet, it detects and downloads them automatically.
07# Automatic updates-Ubuntu provides automatic updates through software sources for all packages installed through built-in package managers. Also, you can upgrade to a new Ubuntu version online when available.
8# Software sources-Ubuntu community provides users with most available softwares through software sources. To install any of them, you just have to open the Synaptic Package Manager, check on the desired packages and select 'apply changes'.
9# Wine-In case you want to use some Windows .exe programs in Ubuntu, you can do so using wine. Though not very stable, you can install and run many small softwares on it. I have also seen guys installing Microsoft Office 2003 in linux this way.
10# Free-Almost all the things in this OS are free.