Have I told you I love you lately? : )
There are certainly property value benefits from living in a historically designated home - no matter how much people complain. Historic Morningside is one of those areas. If we study property values of Morningside in the past 10 years, it would blow people's head right out of the water, but they are one of those communities that do it right.
Even in areas like Miami Shores (that also make me bitter) - historic homes sell for more than others (even if not designated) - same applies in Coral Gables.
You pose a good question about what the criteria is about designation and of course many of the "creative improvements" are left to a very subjective interpretation where beauty is in the eye of the beholder and in many cases the beholder is whacked in the head.
National designation is much tougher with proving historic relevance not just date home was built. In miami-dade, however, the home needs to be 50 years or older to be eligible for historic designation. I've seen butchered homes designated and then new home owners taking the time to undo the wrong and restore correctly, others just sit there awkwardly waiting for the right hand.
I've also been witness while showing historic homes of people talking about removing priceless historic features from the home and I've never been able to sit idly by - instead take my time to explain the historic relevance (maybe costing me deals, but I couldn't live with myself otherwise).
I'm not familiar with a lot of the inside intricacies of each particular city - but yes, it's all about value. It would make a really interesting study to see how different areas have increased in value (even with market downturn) because of their historic architecture.