You can already describe URI parameters in structured way with the new API Blueprint format. Refer to https://github.com/apiaryio/api-blueprint/blob/master/examples/7.%20Parameters.md for details.
The new format is now default and URI parameters are properly rendered in the documentation.
We are still working on description of http message–body properties (parameters). It is planned in the upcoming revision 1B of API Blueprint – as discussed in https://github.com/apiaryio/api-blueprint/issues/25Michal commented
Well, I consider describing input/output attributes as a must-have and after hearing so much about Apiary, I'm quite surprised it's just not here :-/
What I need is to describe each parameter, it's type (varchar, integer, boolean), give example values etc. Just the basics.