But however fine-tuned the system, no point system is immune from gaming, and no-one can expect points to be any more than vague clue as to someone's true worth as a participant in the community.
You don't get a point simply for posting a comment: all points come from upvotes - downvotes. Initial "1 point" label is a purely cosmetic thing.
I think moderation is important, but that it should not be the central aspect of user interaction.
Slashdot's point cap is the obvious solution, but even there comments tend to lean heavily towards +5 on well read threads.
Re: first problem: I think a large number of fairly good comments probably IS a more valuable contribution than a small number of very good comments.
The second problem is more, er, problematic, but you could argue that good comments are popular articles deserve more karma, since these are comments on topics which people are really interested in – i.e. valuable insights on hot topics benefit more people than valuable insights on obscure topics, and are therefore more valuable contributions.