It's been said long and often that the two fatal flaws of John Jaso were that he's bad at catching and that he can't hit lefties. I can't definitely refute the defense one. I do think that Jaso is a sub-par defensive catcher. His numbers seem to convey that. However, the numbers that are available also convey that he's not a disaster back there, merely mediocre.
The hitting lefties thing though. That deserves a more thorough examination. Here are John Jaso's career platoons.
John Jaso Can't Hit Lefties, Except Maybe He Can
Lookout Landing | Jan 16