After suffering a mental breakdown a depressed businessman attempts to rebuild his life with his estranged wife, but when he meets a carefree barista he begins to question what it takes to truly be happy.

Agreed!

Also - there's almost nothing less enticing to me than the thought of a movie about a character who begins to question something.

I know this happens in just about every movie, but it's usually something that happens in conjunction with a much more interesting story. What specifically is the character's goal once he meets the barrista? Does he try to avoid her? Does he stop spending time with his wife to spend time with this new woman? Do they listen to the Shins together?

Agreed!

Another possibility would be to flip genders here. Have it be a depressed business woman meeting a charming barista-guy at the local Starbucks.

If you keep the MC as a man, you could defy audience expectations by having the wife actively fight for her husband, or by having the barista turn out to be engaged already? She may be everything he thinks he wants, but not really what he needs.