Quite often I read a comment thread, and then decide which comments I vote up.
As soon as this is more than one comment, I get an error message
that I can cast only one comment vote every 5 seconds. -- This happens quite
frequently, while I don't remember ever having hit any other rate limitation
of the site.

I think comment votes are quite useful to quickly find the important comments,
and are rather used too little than too much.
So wouldn't it be possible to make the comment vote rate limitation a bit more ergonomic, e.g.
by setting only a maximum number of comment votes per minute, per hour or per day? --
Say, hitting a limit like 6 per minute or 60 per hour would already be much less likely.

<b>Update (August 18, 2014):</b> Now after two weeks, the score of this post
suggests considerable interest in the matter, but nobody proposed anything concrete.
Therefore let me make a few concrete proposals for polling -- see below.
Vote up every proposal you support, and vote down every proposal you are against.
Also feel free to add more alternatives. Depending on the outcome of this poll,
I will make a concrete feature proposal.