No More fsck
Recently did a clean install of Debian on a box and went with Ext4 file system on '/'. I read a while back that Fedora was shipping with Ext4 as default. Ubuntu being more conservative, stayed with Ext3.
I've had a few bad experiences in the past with exotic and novel technologies, I now take a more cautious path. Sometimes.
When I read Google was upgrading to Ext4, and hired the guy who played a main role in the development of Ext4, that was enough to persuade me. If it was good enough for big 'G', it was good enough for me.
One immediate benefit of Ext4; no more fsck. Those times when you reboot your machine and get the message:
/dev/sda1 been mounted 28 times without being checked, check forced.
Sit around twiddling your thumbs while fsck goes to work.
Ext4 is reputed to be faster than Ext3. Read the paragraph on the bitmap allocator.
In the meantime, check out the Ext4 Wiki page, which is packed with useful and meaningful info on the topic.