If the code is "fine" someone should say so (as an answer) which can then be voted up (or down) by the community. Why force users to only provide criticism where there may simply be none to be had?
Knowing when to say "That code is done, move on" is just as important a skill as finding fault.
Part of a code review is verifying the implementation and correctness of a code block. Code review is an iterative process. How will users know when their code is "done" if praise is verboten? The suggestion that "It's fine" posts should be discouraged makes little sense.