Most problems you'll encounter when starting WebLogic Server will be attributable to your classpath. To avoid any problems with your classpath, clear or clean your system classpath prior to staring the server. I suggest that you do not set this environmental variable manually (Windows) and allow your start scripts to set all values. Another option is to allow the WebLogic settings to be first (first class wins in Java) in your classpath before any other items you want to include within the classpath.
Other common problems deal with starting managed servers once your Admin server is initiated. If you encounter problems starting your managed servers, verifying the configurations using your Admin Console is the BEA preferred method. You can also inspect configurations within your domain's config.xml file; however, any changes made there directly must be done with extreme caution. Ensure that there are no conflicts among server names or IP address/port designations. Also verify that your Admin server started cleanly; that is, there were no errors (log files can be found under domain and individual server directories). Note: An RMI error (RMI services not initiated) prevents any managed servers from initiating, even though the Admin server started successfully.