I've advocated pretty hard for this feature and it turns out this used to be a thing but was removed way back in the day (years now). There is somewhat of a movement surrounding this feature, and Github wants to hear more support from the community before moving forward bringing it back. If you support this feature request, please send an email to support@github.com expressing your support for it. For me, I hate that projects will lock-to-contributors a popular issue they don't want to implement because of the +1 comment spam. I can think of two very huge projects that did this in the last couple of months. Github definitely needs a thumbs up / upvote mechanic.
That would disallow you from voicing that opinion so succinctly.
Instead, you would have to add a line that either gives more depth to your opinion (why do you think it would help?) or that just serves as use,es filler. I bet many people would opt for the latter, as it is easier to write.
Unfortunately, I don't think we have technology to measure the amount of content in a comment.
I'm not sure about that. If you take a look at (for example) the awful change on Stack Overflow made by Shog9 to mitigate short comments (and what he believes are therefore bad comments), you'll see the huge backlash and also notice how many people can and do leave thoughtful comments with only a single line or less. I think it is an impossible problem to judge the quality of content by its quantity. The +1 comment is an extremely niche type of comment "problem" that can be mitigated successfully by outsourcing the functionality to a simple upvote widget, but I think that's as far as you can get in judging comment quality without making very broad statements that quash some quality content by mistake.
Honestly, Shog9 is the single biggest reason why my participation in Stack Exchange has dwindled to prettymuch nothing. It's as if he wakes up one morning and decides "I know! I'll make a huge change to Stack Overflow that will piss thousands of users off! ^-^"
What's just as annoying is that as far as I'm aware, he has never admitted that he's been wrong about anything. Ever.