40k 1st edition (Rogue Trader) was originally envisioned as a role-playing game as much as a tabletop one. That's one of the reasons there's so much background material. Being 25 years old helps, too. That's a long time for an (increasingly large) group of creative minds to be adding to a story. One of the reasons for the background is that it makes the sculpts more believable/interesting if there's a rational reason for all the decorations/details on a model. A good example of this is the Eldar. In the original 40k book there was a couple of pages about them. They were basically pirate elves in space. Their appearance was similar to a fantasy elf wearing a slightly lumpy looking bio-ish armour. Some time later one of GWs sculptors was asked to develop the range a bit and make some more eldar models. Now, fortunately for us Eldar players, that designer was Jes Goodwin, a man with an obsessive eye for functional detail. Rather than pick up his sculpting tools and get to work, he sat down and thought long and hard about what kind of people he wanted eldar to be. He made up a whole culture for the species (this is all in a book called the Warhammer 40k Compilation, which is long out of print. You can google Jes Goodwin Eldar concept sketch for some nice pics, though). The point is, once he was done thinking out the culture and lifestyle of the eldar, the design part was easy. Why do their soldiers all wear completely different types of armour? Well, that's cos they have to compartmentalise their thought processes or they go bonkers. What's that gem on ther chests that they all have? Ah well, that's the stone for ther souls. Etc, etc. A well thought-out backstory gives life to a design.
incodentally, the posts from dakka about broken gene-seed marines miss a fundamental point. If gene seed doesn't take properly, the subject dies. Painfully. The implantation of all the organs takes years and years, and is accompanied by very hardcore training and pain-management. It also requires a highly skilled surgeon and high tech facilities. Space marines are made from the genes of the Primarchs. The Primarchs are made from the genes of the Emperor (probably). To interfere with the gene-seed in any way would be a terrible heresy, and would probably result in the perpetrator's planet being virus-bombed from orbit if ye imperium ever found out. One the the imperium is not is Tolerant. Of anything. At all. It is not a nice place.The very slightly aberrant gene-seed found in chapters such as the Blood Angels (slight tendency to fangs, a bit blood-obsessed. Somewhat insane) and Space Wolves (hairy, fangs, occasionally turn into werewolves) is only tolerated because a) the chapters involved keep it very hush-hush, and b) it's quite handy to have an insane vampire/werewolf on your side in a fight.
That said, the chaos marines can and do attempt to steal gene-seed from loyalist chapters. I guess in the eye of terror (the nasty daemon-infested section of the galaxy where the CSM live) they don't have much in the way of resources. There's a rather good series of stories about both chaos marines and ultra marines by Graeme McNeill which has a lot of stuff about this kind of thing. Some of it is from the POV of the CSM, some (most) from the POV of the ultramarines. I forget the names of the books right now, but I'll check when I get home. Maaaaybe it's Storm of Iron, then the Ultramarines Omnibus (book 1). Incodentally, the Eisenhorn series by Dan Abnett is a great read and focuses much more on daily life in the cities of the imperium.