A hard one to answer – almost as hard as the question what is an informative title - but some generalizations are possible. One way to look at this is to consider what the funding agencies regard as worthy supporting (disregard EU). In this respect, the advice circulated in a post to evoldir yesterday, may provide some clues. Although this advice by Joan Strassmann is specific for NSF pre-proposals, I think it is written to effect which is pretty general and useful to illuminate the question above, with several links to equally illuminating essays.
Read the post from here