Have looked far and wide for info and have found no definitive list. Please add your observations. I'm sure it will come in handy to all.
It doesn't have the Nitro Javascript engine. This makes executing Javascript much slower in UIWebView compared to Safari.
http://www.tuaw.com/2011/03/18/apple-confirms-some-webkit-optimizations-unavailable-to-ios-apps/
http://ariya.ofilabs.com/2012/06/nitro-javascriptcore-and-jit.html