The thing about datacenters as opposed to hosting providers is that there is an extremely large amount of money required to operate on a monthly basis. One of the places where many data centers cut corners is on top quality staffing and infrastructure maintenance.

The statement of a "large vs. small" data center is hard for me to agree with. Since data centers exist for adverse conditions (power outage, bandwidth cut, etc.) it is extremely important that your data center provider have the means to invest in active maintenance contracts for their infrastructure (UPS, generators, etc.).

Those are the factors that are most important in regards to a data center. The best recent example I can think of here is SAGO Tampa's prolonged outage at the end of 2007. A power fluctuation led to a cascading generator failure, etc. Those things could have been mitigated with active service contracts.