Why YSK? Comments you reply positively to, should definitely get your upvote. Comments you disagree with should be at your discretion. Trolls deserve downvotes (seriously, they live for those). Disagreeing with someone in the midst of a good discussion doesn’t necessarily warrant one, and might deserve an upvote. Even if you don’t reply, but you agree with the comment, give it your vote.
Also, this has nothing to do with propping up folks’ egos. Comments with more upvotes will likely be seen first the longer the post is up. Alternatively, downvoted posts are less likely to be seen unless users are looking for them.
Of course, this can lead to folks accusing communities of having leftist/right-wing bias, but I think overall it improves the usage of the site. Personally, the thing I liked most about Reddit was the conversations in the comments. Usually the ones with the most upvotes were more worth the read & engagement.
This one is harder for me. I’m one of those people who posts an article without necessarily also writing my thoughts in the “body” box when I post it. Sometimes I do, but oftentimes I think that people don’t want to hear what I have to say about this topic; they want to hear what the article writer has to say about the topic. I try to hedge on this by writing a summary of the article in the body box if there isn’t a summary already provided. I try to save opinion for the comment section.