There are so many things that we look for in a data center when picking a location.
As Ernie suggested, redundancy is a key role in any market, we want dual power feeds, UPS's, Generator backup, the whole works. The number one biggest issue we have had in this industry is hardware failures, those happen most often when a system loses power abruptly. This can also cause data corruption which can lead to hours or days of trying to recover.
Security is another key factor making sure that no one can gain access to equipment without our permission.
Confident staff / remote hands. Nothing is worse than having bad remote hands from the data center. We need to have basic tasks such as reboots, cable swaps, hot swaps etc a free or cheap charge if it's not free a low $10-15 fee for those types of tasks. On top of this we require data center techs to have knowledge of servers and systems this is rare and doesn't happen in a lot of data centers.
A decent amount of network providers, not everyone needs to be in a peering hub with every provider out there however the more options you have the better pricing and options you can have. Downside to this is you normally eat a lot of high cross connect fees from this as that's how some data centers make their money. If it was up to me, all cross connects should be free with a one time setup fee, or a low $25-50 fee at the end of the day once the cross connect is in place they don;t require any real work so paying $150-350 a month for it upsets a lot of people.
This list could go on for a few pages however those are a few major things.