Big difference. Two completely different animals.
The folks that are buying houses (mortgage at 95-100% appraised value) with little or no money down have no real risk other than their reputation. A foreclosure simply means eviction and a major hit on credit rating. Little or no monetary loss. I know of several young couples who have done exactly this or simply quit paying on a contract and let the property default to the lessor/owner. However, it can get real ugly with double rents for a period of time.

Where folks are getting pinched is having a 95% mortgage and an adjustable rate. If they could barely afford the property to begin with and were living paycheck to paycheck, and the interest rate goes up just a leeetle bit, they will have real problems meeting obligations because it busts their budget. Bad decision = bad outcome if the downside risk rears it's head.
Twist. If the loan is for say, 125% of the appraised value and the buyer defaults, they still owe the difference between a sales price if forced to sell and the mortgage amount. They have a choice of paying or filing bankruptcy. While the loss can be significant, the banks/lenders do not really want folks to default and will usually work 'a deal' if individuals are otherwise solvent and working.
Yeah, you could also get hammered by a balloon payment coming due and not being able to afford or get a fixed mortgage. More POOR decisions and planning.
Losses in the housing situation come about when housing values drop
significantly ACROSS the country. That has not come to pass. That's what the media is trying to
make happen with their fear mongering. Some places (coasts) have seen prices flatten for a while, but they start rising again. Pockets of drops, maybe SMALL ones. The economy is so robust right now that this 'housing/lender problem' is being absorbed. It is just not having much, if any, ripple effect on the economy as a whole.
No one should feel the least bit sorry for someone who made a bad financial decision and got in over their head on a tight mortgage and is now forced to default when the rate goes up a half point or more. If someone looses a job and ability to pay for some reason beyond their control, that's another story and THAT happens all the time.
I have held an ARM before and it worked real slick for us. It had maximum annual amounts that the interest rate could rise. It was a half point IIRC and a quarter point every six months with something like a two? month warning of an increase. So even if the prime rate (ARMS are usually tied to prime somehow) went up two points in a year, all I could expect to see is a recomputation based on just a half point. The way I had it figured, we would be ahead of the existing fixed rates for at least three years! We sold for a profit BEFORE that time frame hit. AND we did not max out our budget when we bought so we had 'room'.
I would suggest that commodity prices such as metals and now grains, along with energy will cause some shakiness, toss in govco printing more money and the ensuing inflation, devaluation of the dollar and now he recipe for real problems take front and center.
Margin with stocks, futures and options is a whole different kettle of fish. The short explanation is the brokerage house
loans the investor money for up to about half the value of the stock (This is all predetermined based on your individual net worth and trading limits) when it is purchased. While the investor holds the stock, interest is accrues on the margin account. Any number of ways to pay. So if an investor buys 100 shares at 100dollars, thats $10,000 worth of stock (ignore trading fees for this example). But the investor only has or is willing to put up $5000, and buys the remainder on 'margin'. Then instead of going up, it goes DOWN next week to say $20. The brokerage house sees this and makes a 'margin call'. So the investor has to come up with $5000 plus interest. He sells the stock for $2000. But he must pay the brokerage house $5000. He has to come up with another $3000 and interest to be square. Now the investor is in for his original $5000 + $3000 and interest, for an $8000 net loss.
I am an individual investor and I NEVER TRADE ON MARGIN AND NEVER WILL. It ain't necessary, IMO.
During the crash in '29 folks were on margin in multiple trades and accounts for a lot more than 50% of the purchase price. When the stock went below a predetermined value relative to the purchase price and the amount on margin, the brokerage houses 'called margins' and folks lost EVERYTHING.