The majority of computer users don't spend much time thinking about the file system their operating system uses. Granted, people installing alternative operating systems might give more consideration to the available file systems than the general population -- unless there's a specific need for a certain file system, many go with their distribution's default option.
While your file system type isn't something you should need to think about regularly, and making an informed choice for machines with specialized, dedicated tasks is important, usually choosing the default file system type for your operating system is fine. Several Linux distributions (including Fedora and Ubuntu) are planning to move their default file systems from ext3 to ext4 in upcoming releases, while a number of distributions already (or will soon) support ext4 as a file system option.
IBM has a (rather surprisingly) interesting look at what's different about the ext4 file system, including details on why some changes were made, and how moving from ext3 to ext4 works.
