Have a look at the official <a href=”http://download.oracle.com/docs/cd/B19306_01/server.102/b14238/toc.htm”>Oracle® Database Upgrade Guide</a>. Section 5 covers “Compatibility and Interoperability”.
Here are some other useful links:
<a href=”http://www.dba-oracle.com/t_slow_performance_after_upgrade.htm”>Oracle 10g Migration: slow performance after upgrade to Oracle10g</a>
<a href=”http://www.dba-oracle.com/t_10g_migration_tips.htm”>Oracle speaks on 10g migration tips</a>
<a href=”http://www.dba-oracle.com/t_sql_merge_join_cartesian.htm”>Merge join cartesian tips</a>
Additionally, you might want to take a look at a recent question/discussion here on ITKE, which revealed the existence of some PL/SQL bugs: <a href=”http://itknowledgeexchange.techtarget.com/itanswers/oracle-9i-to-oracle-10g-upgrade-help-needed-for-some-plsql-errors/”>Oracle 9i to Oracle 10g Upgrade – Help needed for some PL/SQL errors</a>