The approach is interesting and can be compared to FIT or FitNesse. Where FIT and FitNesse let analysts/testers give examples in the specifications documents as a series of inputs and outputs (in Word documents for FIT, and wiki pages for FitNesse), Concordion lets you write the examples in HTML, expecting tags to mark which value is for what purpose.
The greeting for user <span concordion:set="#firstName">Bob</span>will be:
<span concordion:assertEquals="greetingFor(#firstName)">Hello Bob!</span>
If you feel that this is making life hard for a large amount of values, fear not: there is a way to use HTML tables as well.
I think that the approach is interesting. It certainly helps making natural speech useful for integration tests. That said, I cannot really imagine analysts or testers writing HTML pages with tags. Word documents (or HTML documents edited with Word) or wiki pages seem much more plausible. Another thing is that the tooling for adding the appropriate tags seems limited. Can we really expect these people to add HTML tags (that is, they cannot easily use a wysiwyg tool for that), and give them values with such constraints? We’ll see with time, I guess.