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.
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.