The fact it's server based and gps based means they can do some different things. They can go a bit more 'area' based and be a bit more comprehensive than street pass. With streetpass the range is presumably quite limited, so you may only pick up a small subset of people you more immediately pass. It has no idea of where you bumped into people, just that you did...so you get no geography info. With this, it could also theoretically show you every user that showed up in a specific area (although obviously that would be silly, but they can show you a bigger picture). It could also be invariant with time, let you know if friends came into an area at a different time after you or before you. It can also give you game popularity stats based on everyone who was in that area, not just based on devices you passed immediately. So stuff like that, basically...it's much more flexible. You could see, like, the most popular games currently among people who frequent your local starbucks. Now, maybe that seems a little pointless, but the whole idea is discovery and trivia I think, and it's an example of the kind of data you could pull out of this.
edit - another thing...does gps depend on 3g? If not, you wouldn't even need to leave your 3g on while walking around to take advantage of these features, which would save battery. The system could poll for gps data every so often and just record it locally, and then you could elect to pass it up to the server later - either with wifi or 3g. All the data it needs to do the necessary 'reconstruction' of your walkabout will be there, it doesn't actually need to be pinging the server as you go.
