Should you own any of the following hats in TF2, you'll now be able to wear them in Portal 2 too:
- Mann Co. Cap
- Prince Tavish's Crown
- Pyro's Beanie
- Fancy Fedora
- Master's Yellow Belt
- Tyrant's Helm
Portal 2, if you were unaware, has character customisation for the two co-op bots. Players can equip Atlas and P-Body with new gestures, paint patterns, accessories, and, of course, hats. Some can be unlocked with achievements, while many more are sold through the Robot Enrichment program--essentially a Portal 2 version of TF2's Mann Co. Store.
Angry, reactive and foolish corners of the Internet, this is your cue to proclaim that hats have ruined Portal 2.
And honestly, do read Valve's blog post. Cracking stuff.