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+>> A very generic metadata framework could cover all possible usages of fields, tags, and related metadata, but keeping its _user interface_ generic would only make it hard to use. Note that this is not an objection to the idea of collapsing the fields and tags functionality (at quick glance, I cannot see a real difference between single-valued custom tagtypes and fields, but see below), but more about the syntax.
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+>> I had thought about the `\[[!tag type1=value1 type2=value2]]` syntax myself, but ultimately decided against it for a number of reasons, most importantly the fact that (1) it's harder to type, (2) it's harder to spot errors in the tag types (so for example if one misspelled `categoria` as `categorica`, he might not notice it as quickly as seeing the un-parsed `\[[!categorica ]]` directive in the output html) and (3) it encourages collapsing possibly unrelated metadata together (for example, I would never consider putting the categoria information together with the rubrica one; of course with your syntax it's perfectly possible to keep them separate as well).
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+>> Point (2) may be considered a downside as well as an upside, depending on perspective, of course. And it would be possible to have a set of predefined tag types to match against, like in my tagtype directive approach but with your syntax. Point (3) is of course entirely in the hands of the user, but that's exactly what syntax should be about. There is nothing functionally wrong with e.g. `\[[!meta tag=sometag author=someauthor title=sometitle rubrica=somecolumn]]`, but I honestly find it horrible.
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+>> A solution could be to allow both syntaxes, getting to have for example `\[[!sometagtype "blah"]]` as a shortcut for `\[[!tag sometagtype="blah"]]` (or, in the more general case, `\[[!somefieldname "blah"]]` as a shortcut for `\[[!meta fieldname="blah"]]`).
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+>> I would like to point out however that there are some functional differences between categorization metadata vs other metadata that might suggest to keep fields and (my extended) tags separate. For examples, in feeds you'd want all categorization metadata to fall in one place, with some appropriate manipulation (which I still have to implement, by the way), while things like author or title would go to the corresponding feed item properties. Although it all would be possible with appropriate report or template juggling, having such default metadata handled natively looks like a bonus to me.