Windows tends to be bad for locking everthing in sight. If program A has a file open, then B often can't even read it.
As to interoperability:
There are two possible approaches generally to photo management:
1. The program has to do everything -- import, tag, edit, export, track.
2. Users assemble their own work flow with a set of tools.
The problem with approach 1 is that different people have different needs. Aperture has pretty good, but not great editing. If you want more, you have to export to photoshop, then import it again as a new image.
Aperture keeps it's tags in alphabetical order, and on export, you lose the hierarchy.
Star ratings in aperture get lost elsewhere.
The advantage of approach 2 is that people get the tools to fit what they need to do. For any given product, the greater the number of programs it can interoperate with, the better the chance it will fit into some one's workflow.
It's moving target. And you won't be able to work with everyone. However the following steps make it easier for everyone:
1. Have a published standard for how you do things. This should be detailed enough that a good program can read and parse your datafiles. PM AFAIK uses a pretty standard XML format for this.
2. Adopt standards as quickly as possible. E.g. IPTC extensions, PLUS,
3. Provide conversion tools to get that data to/from other formats. Example: XML to Comma Separated Variable (CSV) or excel.
4. Provide mapping tools to cheat so that you can repurpose fields for other things if necessary. (Yes, now you are doing non-standard things -- but when the standards don't work, you have to improvise) This already happens on a wide scale. Newspaper and magazine use of metadata is quite different.
5. Provide web page resources for how to get your product to work with other programs. This will be a collection of recipes and gotchas.
6. Provide a list of niche products that would make life easier. For example, a command line tool that would extract star ratings from Aperture, and update ratings in other programs. In some cases, no such program exists.
By providing the interop guidelines, you gain credibility (and customers) in the field.
I too am building a work flow. And I have a fair number of unsolved issues:
* Tracking derivative images. Aperture does a partial job with it's concept of master and versions -- but it has no mechanism for different resolutions of an image. Nor have I found a way to track where you used a given version. It also doesn't do well with external editors.
* Finding similar and duplicate images (Photosweepr)
* I'd like to be able to use controlled vocabulary for fields other than keywords. E.g. for location, and for people.
* I'd like to be able to exchange data with facebook. Extract people tags from forums, and use them as meta data. (I'm working on an alumni project -- I don't know all the alumni...)