Linux

Essentially, you're trying to predict the future; it's difficult to be comprehensive about what might go wrong.

Maybe you had hardware failures, maybe you didn't. Maybe some system services left clutter that needs to be cleaned out before they start, maybe they didn't. Maybe someone put in settings that didn't get saved into any configuration file, maybe they didn't. Maybe you had disk corruption as a result of unclean shutdown, maybe you didn't. Maybe the RAIDs can't find all their disks for some reason and won't go. Or not. We know nothing about your systems or the software and data therein. At this point all we can do is guess, and know that whatever we say is 99% likely to be wrong.

Ah, also you need an escallation plan in case you fail to meet your customers Service Level Agreements, or someone fails to meet yours. For example who do you phone if a hardware vendor doesnt come and fix hardware, or if you fail to get a service running....your boss, his boss ? A list of consequences and priorities of services that hosts provide can help you plan what to do first, and what to escalate.

Another thing to consider during a power outage is that you may lose cooling, even if you are on UPS. You should also consider having a thermal sensor that can send a graceful shutdown to your systems.

Depending on the needs of your organization, and expected SLA and uptime requirements, you should consider redundant UPS systems on different power phases, as you can lose an entire phase of power and not lose the system if they are on different phases.

Proper backups should be taken (And to make them proper, a copy of the data, in a recoverable format, should be off site).