I've really like Watsi since the first time I saw it on HN (before you were in YC). I was thinking about it yesterday and I think the satisfying part is the ability to see a case actually get funded. A lot of charities you give money but it is for a somewhat amorphous problem, to "fight hunger" or "help fund education" or "research a cure for cancer". These are all noble causes but it is somehow less personally fulfilling because you can't see any near term changes and just have to tell yourself perhaps your donation helped in some small way. With Watsi you can actually see that you and some other anonymous strangers paid for an actual procedure, and it got done, and it is pretty easy to see the direct connection between your donation and a real, positive difference in someone's life, even if you don't know / will never meet that someone.
It is the same with that IndieGoGo fund the other day to help the Kenyan hacker trying to get to NYC for Hacker School. There is something fulfilling about refreshing that page and seeing the goal met and then exceeded and knowing it will impact their life in a way that is greater than I could have impacted my own by spending the same money on something for myself (or, perhaps, something more directly for myself, since clearly this kind of thing gives me some personal satisfaction or I wouldn't do it, selfish altruism and all :))
It is the same with that IndieGoGo fund the other day to help the Kenyan hacker trying to get to NYC for Hacker School. There is something fulfilling about refreshing that page and seeing the goal met and then exceeded and knowing it will impact their life in a way that is greater than I could have impacted my own by spending the same money on something for myself (or, perhaps, something more directly for myself, since clearly this kind of thing gives me some personal satisfaction or I wouldn't do it, selfish altruism and all :))