In your experience, what are some common issues that arise when NetSuite does a new release? I want to focus my testing on specific potential problem areas.

custom scripts is the main issue. business processes need to be re-evaluated as well. mind you there are other feature released between releases, they also need to be reviewed

For us, it is missing functionality that is in the Release Notes, browser cache issues (after the upgrade is complete), and the occasional Workflow issue that has run perfectly for 5+ years that suddenly breaks.