How many power feeds?
How much fuel?
What kind of power?
Is it in a flood zone? Hurricane evacuation zone?
How much concrete is in the building.
What is the reliability of the IP and the redundancy set up?
Who has the keys? Who can come and go?
Are you open 24/7 365 even holidays?
How do I ship equipment?
What if I need a server re-booted?
What kind of climate control do you have?
If in California... do you have siesmic bracing?
Will my Servers be mounted in a 2 or 4 post open rack or cabinet, or will they be in a secured cabinet?
Do you have redundant routers?
Are you running BGP?
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"If you quit before you've given it 100% you will be very disappointed in yourself for the rest of your life."
As a guess, when you ask any question to the Data Center you are going to probably talk to a sales person and he is probably going to answer the question in favor of the Data Center.
All the suggested questions in this thread are great.
I would only suggest that you also ask the questions on the Data Centers “Forums” that you are considering placing a server with. If the Data Center does not have a forum or has under 500 registered members than that also tells you something.