https://en.wikipedia.org/wiki/The_Third_Manifesto
The difficult problems since that time turned out to be scalability and parallelism, neither of which were addressed by The Third Manifesto. So instead of D, we got Map Reduce and NoSQL.
Who knows what the hard problems of the next 15 years will be.
http://www.dcs.warwick.ac.uk/~hugh/TTM/Tutorial%20D-2013-05-...
Which provides a description of Tutorial D, an implementation of the prescriptions.
Or "seem" -- I have really no idea how that happened and didn't notice it until too late to edit.
D would overcome the object-relational mismatch by making the relational side much, much more powerful. It's really about unifying the ideas of "tables of data are relations" and "relations are logical propositions", so that one could have an über-powerful general purpose language that just happens to also be a natural interface for databases.
D is essentially a set of low level prescriptions that the authors felt would be required in order to achieve such a language.