I think Iain M. Banks did it best with books set in the same universe but often making out how a book participated in that universe — e.g., Inversions — was rather obscure. That is, he never depended upon it.
It was my understanding that publishers liked series, dependable follow-on sales and all that, and that it was hard(er) for new authors to break into/onto the scene unless they had a series behind a first novel.
Finally, the pressure to make thing into trilogies, and trilogies into series really spoils the possibility of a right-sized pair of novels.
