This is a weird one.
My left arrow key stopped working on my imac, and also on my macbook air.
I tried changing the keyboard on the imac, and the problem persisting suggesting to me it is a software problem.
By a process of elimination I have discovered the problem is with Alfred2.
If i quit alfred it works, if I start alfred the left arrow key stops working.
I can't see any settings that I may have inadvertently clicked on..
any thoughts?