Wouldn't some kind of gravity-based storage be cheaper/safer? Just lift some huge weights with excess energy, lock it in place, and then let gravity pull it down to reclaim it.

Both are back in sight these days.

Australian Synchrotron is already using three giant flywheels as an UPS, here is a short presentation. I am sure that you will be able to find enough information to determine if doing this would be feasible on a commercial scale.

http://www-conf.slac.stanford.edu/wao2012/talks/Thu_Aug9/McG...

There are a few experimental installations of these around. I don't believe they are viable for larger scale storage, but fit smaller scale they seem competitive.

I think it's the other way around. Fly wheels can be extremely dangerous. It is really only large scale that they could be competitively safe.

Flywheel systems are currently sold for short-term power interruptions in datacenters: outage -> flywheel power -> diesel generator

Most datacenters use batteries instead.