The current implementation of GiST within PostgreSQL has some major limitations: GiST access is not concurrent; the GiST interface doesn't allow the development of certain data types, such as digital trees (see papers by Aoki et al); and there is not yet any support for write-ahead logging of updates in GiST indexes.
Solutions to the concurrency problems appear in Marcel Kornacker's thesis; however these ideas have not yet been put into practice in the PostgreSQL implementation.
The lack of write-ahead logging is just a small matter of programming, but since it isn't done yet, a crash could render a GiST index inconsistent, forcing a REINDEX.
