I certainly don't have any idea about how the actual technology works, but my concerns are around both voltage and packaging (there could certainly be some interesting solutions to these issues, they're just not obvious to me).
For example, the battery pack in the Tesla Roadster produces 375 volts (source, warning PDF) while the one used in the Prius (I realize not apples to apples) only outputs 201. Different applications will require different specs. Also, if you're building something like a Tesla, you probably want the batteries as low and flat in the chassis as possible (for handling, since they're so heavy). If you want to place the batteries there, making them easily removable/replacable would be quite a challenge.
I just think assuming every single vehicle would have to use the same size/weight/shaped/spec'd battery pack would seriously hamper innovation. As is, we need at least two different grades of gasoline and diesel just to deal with internal combustion engines and they don't have the packaging issue.