Hello Giulio, First of all, thanks for packaging the MongoDB C++ driver!
Now that Debian has moved to MongoDB 2.6+, it would be great if we could upload libmongoclient-dev to unstable (and possibly have mongo-cxx-driver-legacy provide mongodb-dev as a metapackage as well), as at least one package (uwsgi) could make use of it. Please let me know if you need any assistance; I'm also available to sponsor the upload. Cheers, Apollon