Font Awesome is always getting a little awesome-er. So here's what's new in the latest version, Font Awesome 4.0. Have some ideas for new features? Help contribute.
fa-[name]-[shape]-[o]-[direction]
A desire for better icon naming consistency and predictability are at the heart of the changes in naming conventions. Once you know the name of the base icon shape, you should be able to predict the names of variants.
Solid icons as the base variant
Outlined version of previous modifier
Circle under previous modifier
Square under previous modifier
Directional modifier always at the end
Alternative to the original
Horizontal & vertical modifiers
Consistent when strung together
For complete documentation of the syntax changes in 4.0, check out the examples. For the changes to icon names, @gtagliala has put together a great wiki page that documents all the changes.