You need a bit more karma to be able to change the colour of the top bar.
You need a bit more karma to get the downvote button.
I'm not sure whether your average comment karma is used to place your comment on a page.
I understand how it helps the forum work well, but for the user doesn't seem much more than a vanity feature. Being able to flag/downvote is something I really don't care about.
Score on particular posts is a signal rather than a reward, IMO, and, no, if you are expending effort specifically in order to boost your karma score, its probably not worth it.
I'm still not there yet, and I lost a lot of karma because of it. This is a post of mine that got 31 points - https://news.ycombinator.com/item?id=8339667
And it was picked up by somebody with more karma that created a poll about it and gained 71 points on that - https://news.ycombinator.com/item?id=8339900
That was a bit unfair..