I always see a lot of "it worked for NIH" .. or "it worked for NLM"... or "like PubMed".... or "like PubMed Central"... Gosh, the biomed people have great information structure, some great corpora to use for text mining research, a mandate to make the journal articles that result from federally-funded research available, a great digital library with lots of useful articles... should be an obvious thing to extend that to the other federal agencies that fund research. Why isn't it?
I think in the case of mandates, it's absolutely crucial to consider the impact of the patient groups and non-profit organizations that provide support to patient groups. They are a force to be reckoned with in opposition to the publishers. Society publishers and for-profit publishers both spend a great deal of money actively working to fight government open access mandates. It's the patient groups with the stories of how access or lack of access killed a child, a mother, a father, a sister, a brother, etc., that turned the tide. Remember, too, that the NIH mandate had a pretty shaky start.
There's just no equivalent in the physical sciences and engineering.
Also, in biomedicine you have more than 50 years of money going into information retrieval research to make PubMed what it is. People may complain about some aspect of the interface, but there's really a lot to it to make it work like it does.
Lookit, NASA, DOD, DOE, all have large technical report collections on the web that are freely available to the public. They are, for the most part, a bitch to search and rather unsatisfying in the results. Any one of these organizations could have done better years ago with the money they've spent. (FFS DTIC just got worse).They could have taken journal articles written by government employees and put them in their digital libraries - but they haven't. They just put the metadata in.
ERIC, the Department of Education research database has had such up and down funding- I don't even know where it stands - but it was a really useful resource. EPA is just a mess. Transportation had a decent database with reports... Justice has a few different things... but without steady funding over years and some lobbyists to fight the publisher's lobby... well, I don't think the future is that bright.