I agree 100% with your idea of adding value by commenting meaningfully on posts. I've been fortunate to find a Dolphin who appreciates my comments on his work. The trick is absolutely to catch their attention with a meaningful or eye catching comment and then build the relationship from there.

As to what comments I upvote and respond to on my posts? I try to respond to all that aren't flagrant vote begging. If your comment adds to the discussion then it will get a response and an upvote.