First of all you have to make a distinction between "cheaper oil" and severely depressed oil. What we have now is a severe depression in all commodities, not just oil. Oil has lost roughly 70% vs its previous 5 year average, but commodities in general have lost 60%. This is shaping up to be one of the biggest commodity crashes in all of history.
If it was just oil collapsing, it wouldnt be that big a deal. In that case, we would only be dealing with a few hundred billion worth of defaulted debt. The key threat there is that the Fed cannot (legally) buy oil market related securites. But then again, the Fed could not legally buy Fannie/Freddie paper, but it did anyway. I don't think the Fed would get away with buying oil industry debt though. That right there makes this a potentially bigger problem than the housing bust. Many people assumed that the Fed would have their back on the mortgage backed securities, and they ended up being right despite the fact that Fannie/Freddie debt was not government backed debt, it said so right on the prospectus. Nobody is making this same assumption with oil. So, this debt is going to default and there will be no bailout, except from Congress. If it was oil collapsing in isolation, it would only result in a couple percentage points of GDP lost.
But since the entire commodities complex is collapsing, it is a de facto global depression. We are looking at 2 or 3 housing bubbles worth of debt defaults, and none of it can (legally) be bought by the US or European central banks. The worst part of all this is that these oil company assets will be worth next to nothing when they go to auction. So that debt is going to almost completely disappear. The same will hold true across the entire commodities complex. This will be far more deflationary than the housing bust, which was almost entirely papered over by the central banks with relative ease.
As to why commodities are collpasing, that part is the simplest. The distortions created from the printing of tens of trillions in paper wealth have fueled an asset bubble so large that the masses cannot even access these assets. With no access to assets, the demand for commodities will and must fall. When you're spending all your discretionary income on housing and car payments because asset prices are pumped up to the moon, you are left with nothing else to buy. If you cant buy a house in San Francisco, how much are you going to spend at Home Depot? Not much. That dynamic is what is plaguing our entire economy.