Firstly: test coverage. We're currently at around 40% (it's varied up and down about 3% between #Rails / #Ruby versions, which has been interesting!)
I'm not shooting for 100% – I've made the mistake before of making tests too brittle with respect to changes, and at the end of the day 100% lines covered doesn't necessarily mean 100% of functionality covered anyway.
But some of the key models and controllers are very minimally tested, which feels worth improving given the scale of the changes.