In 3.0.0, implementers typically used AbstractTransaction as a starting point for their Transaction implementation. As a result of this change, they will need to extend from AbstractThreadLocalTransaction to get the same functionality as before. Not making this change will likely manifest itself as compile errors in their Transaction implementations.
There is also a AbstractThreadedTransaction which takes a global approach to raising events on commit. This would be useful for those graphs implementing threaded transactions (i.e. for the Graph returned from Transaction.newThreadedTx()..