A while back the Black Swan Real Estate Zine had a post I enjoyed. Typically you will see data center site selection articles about where TO build a facility, but Black Swan approached it from the other point of view -- 10 Places You Don't Want to Build.
My two favorite on their list are #5 - In a building that used to be something else and #6 - in a large metropolitan area. I see a number of stories about converted malls or warehouses that are now data centers, and while I'm sure in 'some' cases the retrofit study / cost analysis made sense, I view data centers as a very purpose-built facility that should not, if at all possible be "fit" into a building that was built for something else. Not building in a large metropolitan area is another one I believe in -- and not just for the reasons mentioned in the article or stories I've personally heard. If anyone is reading this from Dallas -- are your lights dimming? :)
A few days after that blog post I read another excellent point brought up by Dave Ohara on the Green (low carbon) Data Center blog. Dave brings up the error of a single number view vs. the range of performance. With a number of criteria to pour over when selecting a site, Dave points out that most criteria have relationships to other things and that a holistic approach is rarely executed, marrying site, building, IT hardware and software.