Who is to blame for the Doha collapse?

"Almost everyone" is not a bad answer.  But perhaps you would like something more precise.  Christian Bjørnskov has developed an index of blame, based on the degree of protectionism in a country's negotiating position.   He writes:

In total, a number of countries must share responsibility for the breakdown. Brazil and other Third World food exporters probably were too ambitious on behalf of the developed countries, India refused to accept more competition in its comparatively weak industries, and the US position remained opaque while the country for too long hid behind the protectionist positions of other member states. However, the bottom line is that the main culprit - the member bearing most of the responsibility - is the European Union. The problem continues to be that the official policy of the union is controlled by Southern European countries with strong agricultural lobbies - and the policy is therefore rather clearly dictated by Paris. French top politicians have throughout the negotiations ‘protected’ French farmers against cuts in tariffs or support measures - Jacques Chirac and Dominique de Villepin both went on air in national media to ensure their voters that France would veto any liberalization - which makes the country the Global Public Enemy Number One. Yet, another part of the story that needs to be told is that other EU members also made an indirect effort. The EU as a whole and traditionally liberalist countries such as the UK and Denmark in particular are all accomplices.

Colombia, a country with virtually no influence, had the highest pro-trade score.  Bjørnskov does discuss the fact that the U.S. took an "easy" pro-trade position (it knew other countries would never agree), but I am not sure how this influenced his calculation of the index scores.

Thanks to Peter Kurrild-Klitgaard for the pointer.