Not very surprising. I used to vote based on "does the comment have a fair number of points", so a comment with 100 points would very rarely get an upvote from me - whereas now I don't have that information, so I can only use "do I like this comment/do I want to see more like it".
Unfortunately, simply taking away visible comment scores isn't a perfect solution to the problem of voting with the herd.
Even without visible comment scores there are still some indications of how many votes a given comment has received.
Highly rated comments tend to be at the top of the page of comments.
This can have a huge influence, especially in long threads where people often don't have the energy or interest to read all of the comments, so they'll tend to only ever see (and, consequently ever vote on) the comments near the top.
Also, since most everyone knows that comments near the top tend to have more votes, seeing a comment at or near could sway someone's vote as much as just seeing the explicit vote total itself.
The other indicators of how many votes a given comment has gotten are that comments with a vote total of 0 or less are grayed-out and tend to be near the bottom.
Again, this could sway people to vote the comment down, as it's a cue that the community approves of voting that particular comment down.
Like voting on polls, voting on comments would be fairer were the order of comments completely randomized and there was no indication at all (explicit or implicit) of how many votes a given comment had gotten.
Unfortunately, unlike polls, the order of comments can't be randomized without completely destroying the threading feature of HN, and without making the reading of a comment thread incredibly confusing (as replies could then be placed above the comments they were replies to, or not, since the order would be random).
However, even stopping short of complete randomization of the comment order, some of the other indications of comment scores could be removed. In particular, HN could be changed so that highly rated comments no longer tend to move to the top, and so that lower rated comments don't tend to move to the bottom. And, of course, comments with low scores need not be grayed out.
If these changes were implemented, voting on comments would be fairer, but (obviously) scanning through a heap of comments to see which are the most approved/disapproved comments would become more difficult.
PG posted at some point that comments which had a lot of points before actually have more points after the don't-show-points change.