It’s an interesting UX story. My personal theory:
Likes are really personal, so when someone Likes a feature request, they feel like they’re not only expressing their feelings for that feature request, they’re expressing their feelings for that person. It doesn’t have to be about genuinely liking or disliking another person, it’s just that not everyone wants to explicitly “favour” anyone. Likes is also a form of social currency, since it affects Top list rankings, Trust Levels and Bagdes.
This is why I theorised a “Likes Archetype” that could be detached from regular Likes and be made less personal. When you’re strictly voting on a thing and not also on a person, it’s suddenly not a loaded decision any longer.
Reading your topic I also saw that your forum’s old equivalent of “Likes” was "Thank You"s, which will certainly leave an imprint in older user’s minds.
I’d rather say that Likes are largely unappreciated and underestimated as a communication tool. It’s a style of communication that needs to be lifted up by healthy norms just as much as the general discourse on a forum. I believe Meta is quite an excellent example of Likes Done Right. From very early on the co-founders themselves Liked a large amount of posts, indiscriminately yet righteously. Likes are not a rare commodity; it’s a non-zero-sum social currency. Likes begets more Likes begets more Likes.
(p.s., cool feature of Likes: In my reply to you I’m expressing some disagreement, but I still Liked your post, because I think you added value to this discussion. Granted, a user more personally & competitively invested in a discussion might not use Likes that way, which brings us back to healthy norms and setting good examples.)