As a developer I'd like to receive not only negative feedback during a code-review.
As the case of Facebook clearly shows, the platform has to make it easier to express good feedback than negative feedback (pressing a like button is easier then writing a comment) to make the balance between the two more representative of the reality (people with negative feelings seem more determined to express their opinion, so you have to lower the bar for the positive feedback to observe it).
A Code Review is a best practice. But is also a bit frustrating and emotionally depleting experience. Sure, one should be detached from the code authored; sure, one should strive to accept constructive critique, the same way we handle error messages and warning messages.
But wouldn't life of developers be better, if the compiler said "Well done!" on successful compilation, and colleagues could express their awe for a cool line of code effortlessly?
Perhaps, then the profession of a developer would attract and sustain a more diverse range o personalities.
I propose a simple feature in witch hovering over a line, displays a clickable "thumb up" icon, which simply gives a positive feedback to the author.