I was reading the forums this morning and saw a funny post. Because of my time on forums over the years my initial instinct was to "like" their post, but such an option doesn't exist on Completionator. Would this be doable?

Note that I personally prefer when this is implemented purely as a positive ranking. Sites with + and - votes are iffy to me since people may abuse the negative voting at times.

This is definitely doable, it's just a matter of figuring out how to implement it. There are a variety of ways (some of which you mentioned), so we just need to pick the right one for our community :)