Ranking seems relatively cheap when all indexers are trusted. Not so in a trust-nothing peer system. YaCy has already pushed the state of the art of P2P higher than I expected in coming years by use of DHT so obvious in retrospect. It's still just a sort-of working proof of concept with great potential for evulutionary enhancement and reworking. In YaCy indexes can be provided either by website run YaCy servers or a distributed network of crawlers. The former works already for the few sites that run YaCy, and with the publicity YaCy has now, distributed crawling might get close to usable soon.
Yes, ranking is hard - but so was distributed indexing. It became easier. For now ranking has to be done by a trusted site. But perhaps the most important product of the YaCy project may become standardization upon a common protocol allowing searchers to more easily aggregate search results from multiple rankers and for rankers to aggregate indexes from even more crawlers.
If Google refuses to rank Yahoo Mail above GMail, then bohoo. If Google omits a site from their index, then shit. It happens so rarely that their results are overall far superior to those of any other engine. Microsoft has crawled an astounding number of pages, but doesn't yet have all those esoteric pages whose authors have long forgot about and are only linked to by that other esoteric page, perhaps not served as HTML. It doesn't even matter whether Bing's ranking algorithm is better than Google's. If Google's is just good enough to allow their maturity to keep and attract users. And Google didn't exactly stop at the original PageRank.
But YaCy allows decoupling of indexing and ranking, even if ranking practically can not be done in a fully distributed fashion, it allows for ranking to be outsourced far more easily than the current mess of custom HTML soup results. Note that a standardized format for search results (uri-list, RSS, Atom or a semantic HTML dialect) would achieve the same, but such a standard will not be adhered to until a few major players have shown it support for it and classified it as the New Deal(tm).
Disclaimer: I do not run a YaCy crawler yet for a lack of bandwidth. Google is the search engine used by this stable build of Opera, but Bing by the customized experimental build for it can't cope with the insanity that is the latest Yahoo-esque revision of Google's search result page.