Are there any other obstacles you all can think of?
At least within the United States, forgiving debt counts as a taxable event, on the year that the event occurs. This Occupy group has largely sidestepped the issue, and the IRS is unlikely to audit the beneficiaries in this particular case, but the larger the system becomes and the longer-term the system becomes the more likely your patrons will be looked at.. If you're not very careful about how you build the forgiveness, this can even push people into higher tax brackets or out of welfare benefits that they need to live. See some criticism of the Bush-Income-Based-Repayment plan for details on this matter.
Debt collection companies are also likely to adjust the cost of debt around your purchasing power, if your charity persists for long, which may slightly decrease the rate of return. ((Contrawise, there are reasons to keep uncollectable debt on your own books, even if it's obviously uncollectable, due to current accounting practices.))
To expand on your point four, the debt is valued at less than its paper number because these people are the ones that are generally hardest to discuss matters with and/or get money from. This may not always be a matter of income, but a matter of the people not having reliable contact information or living in areas where the courts tend to be very suspicious of debt contracts. Garnishing someone's wages involves drastically different costs depending on your state.
You'd also need to get a fairly good rate of return for this to be the more efficient than typical investment strategies. If eliminating the debt it itself a value thing, that's less of an issue, but it'd be tricky to evaluate that. If you want too much of someone's future wages, they're likely to discount opportunities to increase their future wages (especially due to how marginal tax rates go around the poverty line to four-time the poverty line).
forgiving debt counts as a taxable event
+1
Under the US tax law is someone forgives your $50,000 debt, that $50,000 is added to your income and you owe all taxes on it.
http://www.theguardian.com/world/2013/nov/12/occupy-wall-street-activists-15m-personal-debt
A collection of Occupy activists recently bought over $14,000,000 in personal debt for $400,000.
Normally, debt-buying companies do this with the intention of collecting the money from the debtors--Occupy did not, and I was struck by the lopsidedness of the figures.
A number I see often in the high-impact philanthropy world is $2300 to save a life (with plenty of caveats). At Occupy's rates, that would buy roughly $80,000 in debt--enough to get two or three families out of a hole that would otherwise render them bankrupt.
By itself, this isn't enough to be better than mosquito nets or deworming. But the thing about personal debt is that, thanks to interest payments and stress, it prevents people with high earning potential (compared to an average African) from making decisions that would optimal were they debt-free--like finishing college or buying a used car so they can take on a higher-paying job.
My idea, though it's a tentative, spur-of-the-moment thing:
Why not found a charity that acts like a combination of Vittana and Giving What We Can, freeing people with good prospects from debt in exchange for their signing a contract to donate a small portion of their future salary to charity?
A few issues that come to mind:
1) Occupy bought a lot of medical debt, which this company wouldn't, and other types of debt might be harder to buy.
2) People who have decent earning potential have more valuable debt, since they're more likely to pay it off later. (On the other hand, freeing them of interest payments might help them get into a better position for repayment.)
3) The idea is a lot like micro-lending, and organizations that offer that service don't have a great track record (though some have been successful).
4) People just freed from debt might not be in a position to donate much salary/might be unreliable. (Deferred payments until college is finished/the new job is had could be helpful here.)
5) There might be (well, almost certainly are) difficult legal issues with finding information on people in debt before you actually own their debt.
Are there any other obstacles you all can think of? Other features of the charity that might make it more effective? How does it sound as an intervention that increases the world's productivity in the long run, stacked up against other such interventions?