Ralph Levan has a succinct article in the current Information Technology and Libraries explicating the current profusion of search protocols by placing them on a continuum of complexity. Here is the e-print.

LeVan, Ralph. 2006. "OpenSearch and SRU: A Continuum of Searching." Information Technology and Libraries (ITAL), 25,3 (September): 151-153. Pre-print available online at: http://www.oclc.org/research/publications/archive/2006/levan-ital.pdf (.pdf: 42K/4 pp.)

Incidentally, this article is a contribution to the celebration of Fred Kilgour's role in establishing Information Technology and Libraries.