Some ideas are so superbly good that when you first hear about them you wonder why no one thought of them already.
In the case of RapLeaf Cory Doctorow already did. In Down and Out in the Magic Kingdom he speculates how status will be determined in a post-scarcity civilization.
George Orwell points out in Animal Farm that "some people are always more equal than others". Presumably this will continue to be the case in a post-scarcity society. After all, if there is no measure of prestige, then what incentive do people have to excel?
In Down and Out in the Magic Kingdom a person's "wealth" or prestige is dependent on "whuffie" - a constantly updated measure of the regard other people have for that person, based on a network of neural-implants.
The RapLeaf project is an example of this sort of thing happening on the web. This also has the potential to be genuinely useful. If you meet someone online or stumble on an interesting weblog you can find out what everyone else thinks of them.
OK. From that point of view it might not be such a good idea. For the same reason we have representative democracies. Large groups of people who have not spent considerable time studying proposed laws cannot be expected to vote on them sensibly.
This is a problem with demarchy. But in the context of the web, it would mean that someone would be tainted with a "bad reputation" simply because their opinions don't chime with those of the majority of people who come into contact with them on the web.
I suppose this comes down to whether we can ever really trust "the wisdom of crowds".
I suppose in this context, and as long as it remains largely directed towards private profit-making interests, and as long as the majority of people who use it are reasonable and not there simply to cause trouble (Wikipedia works fairly well, after all...) then RapLeaf could be on to something big.