Potential state tax policy / law changes in response to federal tax reform are starting to look ridiculous. Some proposing a new payroll tax instead of income tax. Creating a charitable contribution fund instead of paying property taxes. Changing the definition of taxable income, etc., etc., etc. Simplicity and conformity at its best (NOT).
Whether it is the individual income tax or corporate income tax, most states use federal taxable income as the starting point to calculate state taxable income. Each state either conforms as of a specific date, or they are a 'rolling conformity' state which means they automatically conform to federal changes. The way a 'rolling conformity' state does not conform to federal changes is by passing legislation to 'decouple' from the federal law. Examples of areas where states have 'decoupled' historically include federal bonus depreciation, and the domestic production deduction.
Each state will be making tough decisions during their legislative and budgeting sessions this year - what federal provisions to conform to and which ones to decouple from. Just like corporations should be doing, the states will be doing a lot of 'modeling' or 'what-if' calculations to determine the tax or revenue impact.
Buckle your seat-belt. It's going to be a bumpy ride.