Lately I've been doing a lot of debugging using the System Output tracing in WebCenter Content 11g. This is built-in tracing in the content server which provides a great level of detail on what's happening under the hood.
You can access the settings as well as a view of the tracing by going to Administration -> System Audit Information. From here, you can select the tracing sections to include. Some of my personal favorites are searchquery, systemdatabase, userstorage, and indexer. Usually I'm trying to find out some information regarding a search, database query, or user information. Besides debugging, it's also very helpful for performance tuning.
One of the nice tricks with the tracing is it honors the wildcard (*) character. So you can put in 'schema*' and gather all of the schema related tracing. And you can notice if you select 'all' and update, it changes to just a *.
To view the tracing in real-time, you simply go to the 'View Server Output' page and the latest tracing information will be at the bottom. This works well if you're looking at something pretty discrete and the system isn't getting much activity. But if you've got a lot of tracing going on, it would be better to go after the trace log file itself. By default, the log files can be found in the <content server instance directory>/data/trace directory. You'll see it named 'idccs_<managed server name>_current.log. You may also find previous trace logs that have rolled over. In this case they will identified by a date/time stamp in the name. By default, the server will rotate the logs after they reach 1MB in size. And it will keep the most recent 10 logs before they roll off and get deleted. If your server is in a cluster, then the trace file should be configured to be local to the node per the recommended configuration settings.
If you're doing some extensive tracing and need to capture all of the information, there are a couple of configuration flags you can set to control the logs.
#Change log size to 10MB and number of logs to 20 FileSizeLimit=10485760 FileCountLimit=20
This is set by going to Admin Server -> General Configuration and entering them in the Additional Configuration Variables: section. Restart the server and it should take on the new logging settings.
Update - Sept. 27, 2012
Kevin Smith has a nice blog post that describes some of these trace sections in detail.