Totally agree with Christie. Speaking only for myself, I'd say appraisal criteria for web archives are no different than any other format. What is needed, regardless of whether something is web-based or paper-based, are clearly conceptualized, articulated, and executed collecting policies whose outcomes (aka collection-specific decision making) are documented upon acquisition/accession.
It is, of course (and were it your mandate), easier to collect the web presence of x-number of organizations than it is to collect the documentary records of x-number of organizations (even in just a rote logistics comparison), but I think that makes the application of institutionally-defined appraisal criteria all the easier, really. In my experience, archives are more likely to "fuzzy" their selection standards for analog collections that "show up on their doorstep." The web isn't going to show up on your doorstep (though how awesome would it be if it did!) -- you have to go out and get it. Which, again imo, makes those selection decision a bit less prone to contingency and thus more aligned with existing institutional practices.