What Would Drupal Do?

Thanks all, I appreciate all the feedback so far.

Workflow is fairly on target with what I'm thinking ... I notice that fago has proposed (on one of the Drupal groups) breaking it up into discrete layers, one of which would be an API layer. That might be the best level at which workflow proper and simple attribute tags might share some code.

Relationship module does a lot of extra stuff, although it does list as one of the features "Pluggable ontologies - a module can create its own predicates and define the behaviour of what happens when a node has (or wants to have) a user-defined property. (much like Taxonomy)" which is exactly the sort of thing I'm proposing.

It's primary purpose though is to do all the other relationships-between-nodes stuff, that I think it might still lead to the same types of support problems -- trying to explain "I know that you don't need relationships, but you can install the module and just use this one little part of it by doing such and such". A lightweight, direct approach seems more sensible and easier to come to grips with for average users.

It does seem like there is enough commonality though that all three and others could do well to work off of a shared, general API for these types of functions. In fact, if it were general enough, it would make sense for taxonomy itself to work off as well, but being in core that makes it more difficult to try to factor this stuff out of the taxonomy.module.