The Grammeen Bank probably changed more lives for the better than all the World Bank loans put together. (That's what the big deal is, Yov.)
It wasn't that no one thought to do it before but that no one was willing to do it before. Banks have long used differential access to credit to control the distribution of property ownership.
The high interest rate is not a big deal when the loan principle amounts are so small and the terms are so short. I believe the average loan size has been around $100 for one year. If you look at the amount of money you actually pay in compounded interest over the life of a 20 year mortgage, it is a much higher percentage of the principle than what these small borrowers are paying.
Microlending has spread everywhere. Lots of non-profit organizations now have a micro-lending arm to their regular activities, and some of these have been bought out by traditional banks because they are so profitable. The repayment rate on micro-loans is about 96%, way higher than traditional debt which, in the third world, runs about 70% I believe. And, like Voronwë said, 96% of the loans go to women, who could not walk into a bank and borrow money in their own name anywhere in the third world where the banks were financed by first world banks. Women couldn't even do this in the US until the 1960s.
Looking back over the so-called development decades of the World Bank since WWII, I am tempted to say that the Grammeen Bank is the only idea to have made a permament improvement in the lives of ordinary people, EVER. The Nobel Prize is certainly deserved, and it's the first time in my recollection that it has been given for what we all normative economics - economics concerned with distributional justice - as opposed to positive economics - solutions to theoretical problems. It's also the first time, I believe, that it has ever been granted for an institutional improvement as opposed to some academic exercise.
Obviously I'm very enthusiastic about this one!
And it's the first time I've ever been enthusiastic about a recipient in Economics. The Nobel Committee should have been hanged for giving the prize to the likes of Jeffrey Sachs, who is possibly responsible for as many deaths by starvation as Joseph Stalin, just not all in one place at one time.
Jn
edit for multiple typos. I am not functioning today
Muhammad Yunus, the first Noble Prize winner from Bangladesh, founded what is known as the micro-credit movement with his Grameen Bank, giving very small loans to poor Bangladeshis who did not qualify for loans from conventional banks. No collateral is needed for loans, which average about US$200 (euro160). One interesting point that I noted is that 97% of the banks loans now go to women. 
