Google Farmer Update: what went wrong?

On the evening of 24th February, Google Principal Engineer Matt Cutts promised that a newly tweaked search algorithm that impacted on 11.8% of search queries (an absolutely massive change to the way that search works, in other words) would "reduce rankings for low-quality sites which are low-value add for users, copy content from other websites or sites that are just not very useful" while at the same time producing "better rankings for high-quality sites with original content and information". Nobody would argue that these are good things, exactly what most of would want our favorite search engine to do, in fact. So what went wrong?