Even before Facebook made the thumbs up (or rather, the like button) ubiquitous, people have been giving that action to indicate that they like you or whatever it is you’re doing or offering. A minor update to the Google Play Store now gives you that ability to give a positive review to an app or game by giving it a thumbs up. But if you don’t like it, you cannot thumbs down it anymore, but you can report it as spam, which is basically not the same thing.
There is no explanation as to why the thumbs down was removed. So now, if you really wanted to say something negative about the app or game that you downloaded (hopefully, you did download it before actually giving it a negative review), then you’d have to actually write a short (or long, if you really want to point out things) review of the app or game. This makes your complaint more objective, rather than the subjective thumbs down.
However, if you really think that the app or game is spam, then you can mark it as spam through the overflow button. But if you would like to affirm the developers that the app or game they created is awesome, then thumbs up it is. It helps increase the visibility of the app or game and helps rank that review higher. That is because the Play Store sorts them by most helpful. And positive reviews may prove to be more helpful yes?
The Play Store previously fixed its reviews section by adding a reviews highlight which lists the most frequently written phrases of those who have reviewed the apps and games. This makes it easier to see what people think of them, although you can still go through all the reviews if you wanted to.