[bantic]

I think the idea of keeping a log of places I've been to is fairly interesting -- especially if there's a good aggregation/search functionality to it, so I can see how many times I go to my favorite restaurant, what places I went to on this day last year, etc. But it's a pretty simple thing to build, and I'd prefer to build it on my own (pay with my time) than use foursquare for it (pay with my privacy). There are a few secondary things that foursquare does that have some potential, though, notably notes from your friends about what to do/eat/see nearby you that appear when you check in somewhere.