If you are looking for this update to have the same serious face-lift in the user interface department as the last updates for the other smartphone platforms (which should help Foursquare have more real world benefits), then look away now.
Those new versions. perhaps reflecting a change in Foursquare's business strategy, are focused on discovery rather than "I am here," taking your location and using their data to find a location around you that you should visit (for example, if you want a coffee). The Foursquare app on Windows Phone is still running in the same way as previously, i.e. you arrive at your destination, then check-in.
There's nothing wrong with this way of working, and frankly it's one that I prefer (there are far too many "we'll help you find somewhere" applications out there already). And here's where the volume of Windows Phone handsets in the market does become part of the equation. There's a cost in time and resources to developing an application, which is balanced by the return from the users.
Still, it is an update to an application that makes really good use of the Metro interface, and you can pick up the new version from the AAWP app directory or update the software on your device through the Windows Marketplace.