The vision and scope for warhammer and 40k belong to Rick Priestly, John Blanche and Brian Ansell. For these, seemingly well educated guys we have much to be gratefull for. Others have done little or serious demage - many writers seeming nerdy and insular rather than outwardly referential as 40k really is (references being medieval history - The Silmarillion, Paradise Lost and many, many others) These guys work for a fantasy/sci-fi games company - but Warhammer and 40K demonstrate a huge love for history and historic wargames (note Warmaster and of course Warhammer Ancients seem to me to be indications of Priestly's true loves.)
Transporting Knights, Elves and Orks into space might seem silly at first, but when constantly re-enforced with cross referencs to real history and culture suddenly it seems somehow more authentic - say compared to Apes in space suits

. But when continually referencing itself it goes in the direction of pointlessness and extreme nerdyness. Star Trek and Star Wars have often made this mistake.
Any way it's my belief that these references contribute to making GW stuff so popular, even if the references are barely understood by the consumers - and the more obscure ones (chaos for example) allow us to dig deep and gives material for our curiosity. And with the internet it's much easier than ever to follow things up.
Size and scope is a problem especially with novels. Dan Abnett can get it about right at times though the real difficulty is with the novel form which focuses on a few characters. History books allow time to concentrate on greater scope by not getting bogged down on the personal matters of a few characters.
Space Marines to me have always been futuristic version of Holy Warrior Knights such as the templars. The idea is that you have a small band of 'holy' brothers who, can control huge areas. The Teutonic States for example. They do so with huge numbers of more ordinary folk of course - more normal warriors, administrators etc. While the historic intent is there and the picture is painted, 1000 marines per chapter seems not enough for the job at hand when we're talking about planets etc. Then again, as a friend pointed out to me: How many men did Cortes have?
My thoughts