1. Real Application cluster improves significantly Oracle Parallel Server, for one the buffer chance of one instance is transparrent to all others. That means a lot in parallle enviroment.
2. Standby can read.
3. Data partitioning is improved
4. Logical Standby via Logminer.
5. If you drop a table, after 5-10' for example you might be able to get it back.
I am going to attend next month 9i: New Features for Administrators at Oracle University. Afterwards I will go for the upgrade exam.