There’s now a link to the connected apps gallery that lets you add apps like Weather Channel and Foodspotting right from your phone. You’ll find it under “my settings” on your profile tab.
The only bug I’ve spotted that appears fixed is one that forced text to run outside of the box on the activity feed when a bunch of check-ins were combined together. Everything fits nicely inside the box now.
As we know now, version 5.1′s “we fixed a bunch of bugs” hid the true nature of the release: connected apps. It’s likely this version is also hiding something that will be released at a later date.
UPDATE (10:57 am): Natalia points out in the comments that venue screens have a new section that shows which of your friends have checked in at a place, and which ones have liked it. Tapping it takes you to a full list, along with how many times they’ve been there.