I always tell people to believe what their eyes and skis tell them more than the numbers. When the numbers are properly used they often tell the same story. And I get most of the "eyes and skis" info about the East from reading reports here.
I should apologize for using a subjective expression like "conditions are crap," which obviously means different things to different people. Most eastern skiers have a much more lenient definition than I do, and I would add that many if not the majority of western skiers (I've already seen admin trending in this direction) have stricter definitions than mine.
Marginal conditions can be put in 2 major categories IMHO: So bad it's not worth skiing (heresy to those of the Frankontour mentality), and those where it's better than not skiing, so I'll do it every few weeks but not every week. I exercise this judgment all the time with respect to SoCal local skiing.
In my "not worth skiing" category I define the following:
1)Very limited terrain open, and none particularly challenging. If I'm going to be restricted to groomers, I don't want to deal with the excessive skier density that is a byproduct of limited runs open. From what I read, this is where eastern skiing was this season to mid-January. Also applied to Mammoth before this Christmas, and in general to Big Bear in most seasons until close to Christmas.
2)Adverse weather that puts the off-piste off limits and also leaves the groomed in unpleasant condition. This is primarily the hard freeze/coral reef Riverc0il mentions. Due to rain incidence this is much more common East than West, and what I experienced at Jay in 2003 after the previous 2 warm days at Stowe and MRG. I avoided Baldy in January 2005 for the 3 weeks following an ill-timed deluge of rain. We've heard reports of a "coral reef" spell in Utah in January 2003, and evidently there were a few similar days after I came home from my annual Iron Blosam trip 2 years ago. Late spring cooloffs at Mammoth aren't pleasant either, but normally in both Utah and Mammoth the agent that drives down the temps is the approach of a new storm that will turn the situation around in a couple of days.
My "worth skiing occasionally" conditions are generally:
1) The areas dependent upon snowmaking and grooming for a decent surface, thus minimal off-piste terrain. Snow Summit is the classic example in my neighborhood, and Stoneham in 2003 was a direct analogy in nearly every way. So I was at Big Bear in December and January, but will not go out there during the 1-2 week intervals between my March destination trips. If the more interesting skiing at Baldy were open now, I would be skiing there.
2) The late season/limited hours skiing at Mammoth. From May onwards I go up there once a month until it closes. These trips are generally better than what I read on FTO from the east in April because the limited terrain at Mammoth includes the interesting and challenging top of the mountain and because of the other available Sierra activities like Yosemite.
I am aware that many of you would define these categories differently. To each his own.