I'm not voting you up or down, I'm replying you, so I
guess I'm in the safe way :-)
Anyway, I just want to make the point that nobody should
be punished by just downvoting. Sometimes you're
interested in an article (or you think it is a good
contribution, or that the tone is ok), others you're not.
Therefore, sometimes you upvote, others you downvote.
I wouldn't dare to say which level of downvoting is ok
for each person, as this may vary depending on the
user experience, knowledge or personality. Thus, I would
be against establishing any kind of limitiation to how
many downvotes a user may cast.
On the other side, I may agree with you about avoiding
the use of votebots, but I feel they should be addressed
separately: they're bad, weather they are used for
downvoting or upvoting.