Not really any, any more. Except that FontForge supports much of the syntax which adobe documents but does not support. Adobe continues to reserve the right to change the syntax of anything they do not currently support.
In November 2008 Adobe made radical changes to the feature file format. This meant that my extensions were no longer needed. It also as an incompatible change from the earlier format. FontForge should (still) be able to parse files written in the earlier format, as well as files written in the new format. It will only produce files in the new format now (actually there is a compile time flag which will revert output to the old format, but I haven't tested it).
File->Merge Feature Info
File->Merge Kern Info)
Info->Lookups, then right click on any lookup (to produce a
popup menu) and select
Save Feature File
You can also generate a feature file containing a single lookup by selecting
that lookup, producing the popup menu and selecting