Greece will either shape up and pay back the money they owe, losing their sovereignty in the process to their debtors, or give up getting loans and segue into a centrally planned economy. Neither are particularly positive for the Greeks but hey, you get what you voted for.
If they do the former, they can manage to pay off their debtors over a very long period of hard work and sacrifice, who can then pay off their own debtors, on and on. If they do the latter, they're fucked long-term (see: Venezuela), and their debtors won't be able to repay the loans they owe to other nations, which reportedly will have a "domino effect" where more and more nations go under because the money they were hoping to get back from their loans will be completely gone.
Imagine loaning your friend 2000$ expecting a 2100$ return by the end of the month; however, instead of doing something with this money, your friend decides to live off it for a while and when it comes time to pay back, he asks you for another 2000$ and for the previous debt to be forgiven--he wants to be bailed out. Foolishly you forgive this debt on the condition that he shape up and get his life together, so he agrees, you give him another 2000$ loan w/ interest, and when the end of the month comes, he again says he does not have the money and wants another bail-out. You tell him to go fuck himself and if he doesn't come up with the money he owes you in another month he's gonna get his kneecaps blown to pieces (jk you just won't like him anymore and will stop giving him any more money.) Your friend (loose use of the term anyway) is pretty upset but understands, he asks for some time to get the money together. You say okay, and give him this time. However, the guy you owe debt to is now knocking on your door asking for the money he lent you, with interest; you tell him that you'll have this money by the end of the month, when your friend pays you back. He says okay and so the end of the month comes: your friend still doesn't have the money and is now telling you to go fuck yourself, you capitalist pig you. The guy you owe money to is now in the hole as well; he tells you that if you don't pay him back, he's never gonna loan money to you ever again.
Thing is, you really need this money your debtor has been lending to you, you are dependent in some way on these loans; if he stops lending you money, you're completely fucked, too. Tensions rise: you start banging on your friend's door demanding he pay back what is owed to you, but he has no money; he has resorted to living in a commune with a bunch of stoners who are broke as broke can be, and they all laugh at you for giving any of them any money you freaking moron. You can't pay back the guy you owe money to; your livelihood is at stake to some degree; you may have to lower your quality of life and get into further debt with someone else, assuming you can find anyone who will lend to a man who won't pay his loans back (check out the lending subforum for what this looks like.) The guy who you owe money to can no longer pay the guy he owes; he, too, is completely fucked. This guy also has a guy he owes money to; that guy, you can probably guess, is completely fucked too. That's the domino effect: everyone's getting fucked because someone down the line was a lazy douche. Not everyone gets affected; some people can survive without the loans, but others are totally dependent on them and will be in a heap of shit if they can't keep getting those loans.
If Greece defaults, other nations may also default, and depending on how much they needed those loans to sustain their way of life (political favors and whatnot; welfare ain't free kiddos), this can be a massive issue for economies and political entities the world over, since everyone and their moms are in debt nowadays. Not necessarily a bad thing; maybe it'll be a wakeup call for people to take control over their money and stop letting politicians and banks play with it. Anyway, someone's bound to die from it all; someone always dies from something this massive.
As the saying goes, if you lend someone a little money, you have power over them. If you lend them a lot of money, they have power over you.
Greece has a lot of power over the EU and world bank, because they have forced Greece into this stupid 'kick the can down the road' idea for the last 5 years, if and when Greece decides not to pay back the debt, they will have to answer for thei sub-prime lending!
I think it could happen today. The whole situation is inevitable, hopefully someone has the guts to make it happen today, instead of after another 5 years of hurt for the Greek people.