My thoughts:

- A log scale is far better than a linear one. While it's not top-end limited, it does tend to flatten the scale markedly.

- A decay function on older contributions is helpful. Especially if users start abusing the system. Bank karma, abuse system, run karma down slowly, with an eternal linear system. With a logrithmic decaying system, abuse degrades reputation far more rapidly.

- Other options are to rank users by percentile or other bases. Given the various problems of innate / explicit moderation / ranking (see A Better Wayto Rate Films: http://blog.goodfil.ms/blog/2011/10/07/a-better-way-to-rate-... ) an explicit percentile norming preserves meaningful distinctions (though individuals ratings may not be directly comparable).