The release notes don’t go into many specifics:-
Say hello to the all-new foursquare! We’ve re-imagined the entire app to make it even easier for you and your friends to share and catalog your experiences and find new places to go. It’s also really really pretty, with tons of fun surprises. Grab the update now!
But after opening you’ll get a quick guided tour of what’s new. In addition to an overall lick of paint you can now see more of your friends check-ins, get recommendations of nearby places based on your activities and add foursquare to your Facebook timeline, when you grant permission of course!
The new look is not complete overhaul, you’ll see be able to find your way around without a problem although the check-in button has moved from bottom-center to top-right. It does look more polished however especially in the Explore tab where finding popular places and your previously visited check-ins is now even easier.