We've found socks and cigarette butts. I guess that flooding by the mains water is seriousl worth thinking about. The anecdotes in this discussion thread, I think, are definitely worth keeping somewhere, although I'm not sure of the venue. You will want to identify possible hazards including fire, flood, etc.
Some buildings have to have large equipment lifted in with a crane or helicopter.
Air conditioning equipment or major power supplies are examples of very hard to handle components. Do a thorough electrical requirmements plan, and be sure the grounding system is up to current practices. If it's a union shop, or especially if non-union personnel will be doing any work in a union area, see if you can get the shop steward on your side. When planning cable runs, be sure you can access critical components that otherwise might be buried under a heavy mass of cables. Plan the runs so they will not interfere with cards sliding in and out of chassis.