There is a website and iPhone app that contain a listing of kosher restaurants. In addition, one feature is the listing of 'star-ratings' and reviews by users. Are there any halachic ramifications to this?
If you feel the restaurant quality is poor, then to tell others as much (so they won't get a bad experience) would be "lashon hara for constructive purpose" (l'toelet), just like if someone asked "do you think this would be a good shidduch" or "should I go into business with this guy?". Just make sure you stay as tone-neutral as possible; that you have no ax to grind; and that nothing disproportionately bad will come of it. (If a restaurant is truly, truly bad, than its going out of business due to lack of customers is probably not disproportionate.) If you're just rating by number of stars (and there are many other raters as well), I don't see the issue. Yes, different people will have different taste, so just because you didn't like it doesn't mean nobody will; but hopefully the online readers will figure that out too. (If it's currently unreviewed and you give it zero stars, that looks really bad. If it has a bunch of five-stars and you give it one star saying "too spicy for my taste", people will read that appropriately.)
I disagree - I will not send my friends to bad restaurant that is bad. If I know the owner and my friends know the owner, then I would be hesitant to say anything bad. If I go out of my way to make sure the restaurant is known to be bad thats one thing, but casually in a conversation where people are talking about "bad experiences in restaurants", then thats the topic and everyone is allowed to express themselves within reason.