Reputation: "just don't do it at first"
As a person who designs and builds social software, I sometimes disagree with but always respect and appreciate the insightful and detailed nature of Christopher's posts. Life with Alacrity is a great sanity check.
With regards to "Reputation"--the only 4-letter word in the English language with more than 4 words--reputation systems (e.g., eBay seller ratings, Slashdot's karma system, & etc.) Christopher wrote something back in February 2004 that still rings true today and may ring true forever,
"Be very careful of the design of rating systems and reputation systems. They are extremely difficult to design well, as they too often can be gamed, or fall into reciprocity such that they are meaningless. My personal advice is just don't do it at first -- save it for a 2.0 version of the product, not 1.0 beta. If you are going to do it now, really study it -- there is a lot of good academic research on issues of reputation. It can be hard to slog through but it is worth it. Offer a grant to Danah's school for them to do research for you on the topic.
Endorsements are a best way of doing reputation for now. They is also is imperfect and vulnerable to reciprocity games, however, as least you can see if two people are playing that game just by looking at the endorser and endorsee. If you find too much reciprocity you can basically ignore both players.
Allow endorsements at a more granular level. For instance, if you are offering dating and professional information, segregate the endorsements. Even more ideally, do like LinkedIn and allow endorsements of relatively small things in a profile, for instance "I really enjoy Chris' blog" against the blog profile item, and "I like what Chris says about the environment" against my affiliation with a group. Consider allowing endorsements of groups: "Of the three groups here on Wiki, I find this one fits me the best".
If you can afford it, consider hiring a game designer from the online game industry who understands behavior incentives in online games. If you can't, study it..."