Squaw takes a lot to get covered, and it had absolutely nothing before Dec. 15. Silverado is the last lift to get open, and it's the only lift not open now. But that probably means that most but not all lines are skiable on KT-22 for example.
MarcC's 80-inch base is a common benchmark for steep, expert-oriented mountains. It's been quoted several times for Taos. I'd certainly want to see 80 inches as the average of upper and lower base depths for Squaw, and since it's now 36-60 I'd say Squaw has farther to go than Snowbird. If you want to ski one of these places (Jackson, Crested Butte, Whistler also) and you want nearly all the marquee steeps to be open, the 80-inch base is not a bad rule of thumb.
In terms or "whether it's worth skiing" or "how likely am I to trash my skis" conditions are not as stringent, and it really depends on the layout of the particular area. Alta and Mammoth are examples where a lot is skiable on 3-4 feet, and it's fairly straightforward to stay out of the places that might damage your skis. There is variability due to wind deposition, particularly important at Mammoth where nearly every storm is accompanied by high winds. So that is indeed a logical explanation for the delay on Mineral Basin, which I suspect would be open in most seasons with this much snow.