This is pretty much an FYI. After returning my controller twice I've released this must be an issue with an entire batch of controllers. All three controllers have had left triggers that feel like they're grinding against something inside. After some fiddling around, I found 3 contact spots where the trigger came into contact with the shell where I presume it wasn't intended too. The trigger actually touched the plastic beside the foam, before touching the small foam pad. After filing these pieces away a little, the trigger responds much better, and more so like right trigger.
As far as I can tell, the trigger button is slightly twisted more that the right trigger, which causes it to contact the shell where it's not suppose to.
Sorry in advanced, I'm only posting this here because hardware forums seem to have vanished and it's difficult to find a way to contact Microsoft. I'm just hoping someone sees this.