What are the most important points to emphasize while pitching for a dissertation to be published as a book? An editor for a major publishing house is visiting our institute, and some PhD students will get the chance to have a 15 min talk with them and discuss the publication of their dissertation as a book.

I would like to pitch my dissertation (a few months from completion) as a potential book for their publishing house. I am in the social sciences in Europe and there is no tradition of the authors getting any financial benefits out of their books, indeed sometimes we have to pay the publishers. But I would like to convince this one to 
a) Publish my diss
b) do it for free

What is the sort of information that this editor would be the most interested to have regarding my dissertation? What important criteria do such editors consider in coming to a decision regarding the publishability of Ph.D. dissertations?

Just to clarify, this is not a formal book proposal but only a short informal conversation, but one which I hope can be useful to both the editor and me.
Editors need books that will have an audience, a point, impact, and a connection to their "lists". 

In a conversation you should be able to briefly summarize your manuscript -- and in this summary make clear its central argument / narrative, scope / evidence base, and its what is novel, important, and exciting about it. You should also summarize the audience for the manuscript,  -- how it fits into the market: what other books are competing for this audience, and what distinguishes your manuscript. These summaries should be honest, and direct -- avoid both negativity and overstatement. Choose language to appeal to a smart, and critical, but not  specialize reader -- avoid jargon, and if you have to use specialized terms, define them briefly, in common-sense terms. 

For a written prospectus, follow the same model, but expand to two pages; and include an outline of the work, list of figures/tables, and note any special features such as data, or an accompanying website. 

For more details, see: Germano, From Dissertation to Book and Luey, Handbook for Academic Authors