When either selecting a data center as a co-location facility or designing a new one from scratch, what would your ideal specification be?
Fundamentally, diversified power sources, multiple ISPs, redundant generators, UPS, cooling, and physical security are all desireable.
What are the additional key requirements that someone might not consider on the first pass?
What are the functional details someone might not consider during the initial high level design?
I'd like to approach this from the perspective of designing a large data center or seeking a facility that was designed perfectly from an infrastructure perspective. This question has already been addressed with smaller facilities and workspace considerations here.
I've been lucky enough to have built a few over the years, certainly I'd look to the following points;
I hope this is of help, I might add some more later.
One thing that I don't see already posted is the budget to be able to able to build a very good team of people.
I recently went cage shopping and found that they pretty much all were peered with multiple tier-1 providers, multiple diesel generators, etc.
What made me pick the one I did was that everyone there was sharp and dedicated, there were plenty of people on location, the sales managers and projects managers were also great. All the generators and peerings in the world won't help if the guys plug you all into the same generator, or the remote hands don't respond when you really need them to.
So this may not fall under infrastructure, but in the end it can be more impressive than four vs two redundant generators, 2 vs 3 Internet peers, etc.
There's good information in the answers to this question. Many of the questions tagged server-room are relevant, too.
Location , power costs, water costs/supply (depending on how the facility is being cooled), weather, and natural disasters is what I would add on top of your list.
Diverse fibre/connectivity routes (including multiple, separate building entry points)
I'm sure that the answer will be "it depends." Are you asking for blue-sky, if you had millions of dollars and were trying to run Amazon? Is there a budget you have in mind?
Ease of expansion is one thing not on your list. If you're renting a cage, how easy is it to add another cage and get the proper wiring between them? If you're building your own, what do you do when you run out of floorspace? Can you destroy offices and expand within the building, can you knock down a wall and make the building bigger?