What I believe could be useful (if this feature is already available then forgive me posting) would be an option to report ignored tags and tag/value pairs which would alert the user that certain features are not rendered by the current style.
I don't think this is doable. There are thousands of tags and tag/value pairs out there which are documented nowhere. Just have a look at Tagwatch (http://tagwatch.stoecker.eu/Planet/De/index.html). Mappers do have a lot of imagination sometimes ;-).
I see it the other way. I *think* it should be doable not to hard. The style matcher has to match the style rules for each element in the osm file. If he could not match any rule then it should printout a warning. I expect them to be thousands too. I don't know if this would help to user. Johann