Transactional DDL is awesome with Rails migrations, it's not a huge thing, but being able to cleanly roll them forward and back when they fail or you screw something up in development is very nice (and certainly beats having to comment out half of your migration and re-run it like you need to do with MySQL).
I mentioned it in the "this could be included but it didn't meet my criteria and here's why" section in the last couple paragraphs. Yes, it's highly underrated, but my focus was just a little narrow perhaps.