Europe's problem is that many of Greece's creditors are themselves debtors on a massive scale. If the value of their assets declines, the only way they can stay solvent is by reducing their debt, and the only way they can pay down the debt is by selling assets, which pushes their price down even further, exacerbating the problem and spreading it to other securities. It's revealing that the ultimate beneficiaries of Europe's rescue are the big banks, not the Greeks, who will barely touch any rescue money before it goes out the window to their lenders.
If you want a thumbnail sketch of what today's global economic system looks like, consider the following: Governments are in debt because investors would rather loan governments money (by purchasing bonds) than ever pay taxes on any enterprise by which they profit.
At the governmental level, this is called "staying competitive," "creating a business friendly environment"; or else it conforms to some "anti-taxes/pro-jobs" theme: it means deferring to business on whatever business wants, or else business will leave, taking "the economy" with it. And what business wants -- what business always wants -- is to maximize profit and market share for investors.
The profit drive, in turn, leads investors to take on debt in order to invest more. That is what BusinessWeek means when it says that "Greece's creditors are debtors on a massive scale." The article explains that "the big global investment banks" have increasingly used "repo financing" which "[gives] the best terms on overnight loans because there's less risk the collateral will lose value." If the banks can't fulfill the terms of the loans, they sell their assets, which, as the European example has shown, include government bonds.
As bonds lose value, governments can increase their attractiveness by promising to pay higher interest rates to investors. If investors still won't be moved, the government will lack financing to pay off debts and meet operating costs, inducing a crisis.
What's significant about the European example is that the bailout isn't aimed at Greece, which has been saddled with even greater debt, but at the class of investors who sold Greek assets in order to recover from bad bets. The bailout merely puts them in a position to return to their previous activity, with European governments being further disciplined to meet their needs -- by redirecting public money toward bigger bond yields, for example.
What this says about our global economic system is that we are all dependent on the good fortune of whimsical, unaccountable gamblers, who must at all times be enabled to make ever-increasing sums of money, lest national governments, having lost the taste for taxation, find themselves without any revenue but that which can be confiscated from the public, all productive enterprise having been consolidated beyond the reach of their will.