I can't think off the top of my head any specific "brand new" feature in Glassfish.
But it does support the current versions of JEE, notably JEE 5, EJB 3, JPA, Servlets 2.5, etc.
GF is more mature and more tested, so no doubt bugs have been fixed. It also has a better web connector via Grizzly, so it should be more efficient as well.
If you're happy and running in production with SJAS 8, I can't think of a compelling reason to upgrade. However, if you're planning new development, JEE 5 and EJB 3 are much better to develop with than EJB 2, plus it will still run your current EJB 2 code base.
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