[jjice]

This may not be the solution you're looking for, but I've found that I've really liked plain text files with some machine readable parts.

One thing I've done is a new line staring with a colon represents metadata, like

    :tags customer,product2

And I've built tools around tag searching. Same thing goes with dates and other misc metadata I may want to use.

It may not be what you're looking for, but I've found it to be a good middle ground for myself.