[jmcqk6]

The biggest thing I can think of is how it defaults to throwing an exception when attempting to return JSON for a GET request. You could disable this easily enough, but I never understood the rationale behind the decision, and it was a momentary frustration every single time. Hopefully they've changed that in MVC 4.

[randomaccount5]

There is an exploit made possible by returning 'unwrapped' json in response to GET's. This details it fairly nicely: http://haacked.com/archive/2008/11/20/anatomy-of-a-subtle-js...