John McTaggart Ellis McTaggart (yes, double McTaggart) was an idealist metaphysician, concerned with the fundamental nature of being and the world that encompasses it.
This McTaggart is only concerned with understanding Umbraco content.
McTaggart uses the Open Calais API to generate tags from HTML strings. Don't think about tags for your content, just hit the button and let McTaggart do the hard work.
Allowing content creators to add their own tags or keywords is fine, except you'll end up with a tag set representing what the author thinks they've written about - McTaggart, by way of Open Calais, analyses the text to return tags that relate directly to the content and can thne be used to generate more accurate search result sets.
McTaggart is a simple property editor - add it to an Umbraco project, create a datatype using the McTaggart editor, add your API key and the aliases of the properties to analyse, and you're good to go.