A "like" button might be okay, but no "dislike". Allow me to give a few examples why.
On YouTube, that damn thumbs-down button is widely abused by trolls. It is always anonymous, which fuels the abuse. You can choose to share your likes, but not dislikes. IMHO, it is not a proper tool for constructive criticism. As someone who has been on YouTube since 2007, I found out the hard way that the vast majority of people will not leave a comment after giving your video a poor rating. If they do, it is often a very poorly written, trolling comment that doesn't help me improve my videos one bit.
After I'd had enough of the hit-and-run dislike button BS, I went back and disabled ratings on all my videos. I also disabled comment ratings. I set my YouTube defaults to have those "features" automatically disabled for each new upload.
Vimeo does not have a dislike button, only a like button. Guess what? They also don't have the troll problems that YouTube has. YouTube is a bastion of stupidity, thanks to unmoderated vicious comments and that damn dislike button.
The MTBR forum has a reputation feature. Until early this year, all reputation was anonymous. This allowed the trolls to have a field day. Here's an example of the typical troll neg rep. It was a thread about motorcycles, and the OP wanted to know how many mountain bikers also rode motorcycles. I mentioned the Kawasaki KE175 enduro that I owned for a few months back in 1988 (I promptly sold that bike and bought more of the pedal-powered kind after noticing rapid weight gain). My post had absolutely nothing to do with riding a bicycle uphill, yet I got this gem:
After a ton of complaints from many posters about abuse of the anonymous negative rep feature, all rep was auto-signed. Now when you leave reputation for someone, that person knows it was you.
I will only give positive reputation on posts, and "Like" on videos. If I don't like a video, I hit stop and move on. It's the right thing to do. Somebody still took time out of their day and put forth the effort to make a video for the public, so I will not insult their work with that stupid dislike button.
If Facebook ever gets a dislike button, I will delete my account.