Environmental Issues -- Don't forget the air-conditioning! There are to parts to this. First, don't forget to factor in and supply the power required by the air-conditioning units that cool the data center. Second, don't forget to factor in the heat generated by the UPSes on the air-conditioners. Large UPSes generate significant amounts of heat and if they are located in the data center, then the cooling requirements will change and may overwhelm existing AC units. Another environmental issue to take into consideration relates to water. When the UPSes are placed, look at where water pipes and sprinklers are located and consider what they may do to the unit.
Fault Tolerance -- Having redundancy is like how fast a person wants to be able to go in a car. It's a function of money! If the risks warrant it, groups install power coming from separate grids, multiple site UPSes with multiple generators and multiple fuel supplies. The risks and service levels need to drive the level of investment and redundancy.
Expansion/Growth -- When purchasing systems, establish a timeframe of how long the current purchase must be in place. Next, establish a projected power growth rate and error factor. This will help both you and the vendor in terms of sizing the appropriate system. Some solutions are very expandable and others either aren't or have very limited expansion capabilities.
Scheduled Reviews -- Establish a review cycle when the stakeholders will get together both with and without the selected vendor to see how things are going. Power needs change and some environments are more predictable than others. The intent of the meetings is to ensure that the power system is meeting current expectations, projected needs and to identify any corrective actions that may be needed.
As you can see, there are many factors that need to be considered ranging from the risks to the potential solutions. An organization that carefully understands requirements and factors in risks, benefits and costs associated with quality power will be prepared for the next crisis.