>>95969491 (OP)Old Grog who played 1st through to 5th ed here. The reaction for many was amazement, wonder, etc at such a complete system and setting in a single (Rogue Trader) book. It was almost an RPG with the amount of story and settings ideas. Most of the ideas where stoloen ffrom other media (2000ad, Micheal Morcock, LOTR, Battletech, Starship Troopers, etc), but this just made it seem more familiar and cooler. Everyone knew what elves, dwarves and knights were, so sci fi versions seemed easy to accept for a tongue in cheek wargame.
You have to compare the release to other sci fi figures/games of the time, most were pretty crap in comparison and either consisted of solely rule publishers with no figures, or a few figure makers with no attached games. Those few that did make figures and rules had VERY small ranges, or a setting that only had a couple of models of a single race fighting another limited group.
40k dumped a much larger range of models and races onto the market in one go, as well as encouraging players to make up new things themselves, use models from other producers, scratchbuild stuff and proxy, allowing players to easily make up any sort of narrative based army or battle they could think of and that appealed to them. I built Ork stompas and battlefortresses out of plastic card, played space marines against Skaven and undead whfb armies, or fought Tyranids against battletech mechs or Dr Who Daleks. The early editions had rules for making stats for whatever you had lying around and we made the most of them.
It all started to change around 3rd edition, when the "you must only use stuff we sell in our games!" and shop and tournament gameplay/product advertising became the norm. Imaginative players are very difficult to find nowadays, with most just playing cookie cutter games and formats and some actually get indignant if you suggest they make up fun stuff themselves. Oh well, different generation, different strokes, I suppose.