What are criteria by which companies evaluate patents?

Firstly you have to understand the reason WHY companies create, then sell patents
a) defensive
b) strategic - whether cross-licensing or growth
c) sword - ie non-practicising assertion entity

So if you lack the in-house expertise, you may acquire patents (or more likely licenses) to
a) create moats around existing businesses, either ancilliary IP, extensions or fill in portfolio gaps, some exceptional companies look for early stage growth prospects especially ex-uni
b) patent pools, licensing warchest, negotiation with peers or standard setting
c) trolls or vampires

So to take some practical examples, when I go onto FlintBox to look at what universities are spinning out, some of the criteria I look for
a) comparative advantage over existing approaches (can be a toughie as novel != necessary)
b) maturity - as in technology readiness levels and how much capital I estimate it'll take to bring it to market
c) likely uptake/demand, I have a select client list and there are specific gaps they want to address which impacts liquidity

As for google, why not ask their venture arm as they are on the active acquisition trail? http://www.managingip.com/Blog/3448859/Googles-patent-purchase-experiment-leaves-unanswered-questions.html