If you're a little strip mall with 20-30 parking spots, this tax may not affect you as much. I would envision it being targeted mainly at the SmartCentres crowd. Maybe you can even put in some sort of an exemption, anything over X number of spots per store, you get taxed on them. That would also eliminate the "well the by-law says we need a minimum of this many, so we had to put in that many". The vast majority of the power centres put in muuuuuuch more parking than what is required by law. THOSE are the guys this tax should target. The little strip mall with 3 stores and 10 parking spots out front aren't where the majority of the money will be coming from anyway.
And residential units should not be taxed. A lot of the condos put in 1.5 spots for every unit, which is a very reasonable amount.