For this pool, I would say that anyone that has more than around 500TH can be considered a big miner at this moment, so if you go to stats page thats around top 15 miners. If they quit, that is where the pool will be at is approximately 20 something PH.
But the miners that pay more than 5-6 cents per kilowatt can start quitting (although Im not even sure that if you pay 6 cents at this moment, the hardware can pay off what it uses), having even smaller amount of TH, lets say starting 50TH. Just because probably most people simply cant pay to get BTC and have to pay for getting it on top of that, it would be easier and cheaper to just buy the BTC, without the worries about the luck.
Miners are used to mine BTC and get a profit on top of that minus out the electricity bill.
So if things dont change soon, the only ones that will be mining are guys that have lots of money to just throw away for electricity (and hope that BTC will cost a million dollars in the future) or the guys that simply dont pay their electric bill and know a way to steal the electricity. Or the Chinese that produce the hardware and pay little to power up their machines, may also be able to stay afloat.
Hopefully the difficulty will drop by a lot soon! If I remember correctly it should go down about 10% in a few days or so. And Im not even sure if that will help to stay afloat, for the most.
That seems to be a pretty fair assessment. I can only speak for myself, which puts me in the mine-at-less-than-break-even-and-wish-upon-a-star category.

Lets just hope for the best! That the BTC will go up and the difficulty go down!