At work we are just starting to write a new ExtJS front end to our app and 2 gurus are doing all of the code reviews.
They are quibbling (my opnion) that i use the wrong type of for loop as a is more efficient than a foreach loop.
There's quite a bit of information on this if you google it. The general consensus seems to be that foreach loops are slower. And it seems to be slower/faster depending on the browser. If your app is very front-end heavy then I wouldn't consider it to be "quibbling".