When I first bought into
40k, it was the 2nd Ed box set. I didn't touch the models for almost a week because the three books included in the box had me gripped. I was a 13 year old boy learning about the far future, and all the awesome weapons that went with it. Hell, I still have the 2ed Wargear book. It's brilliant!
The fluff and general backstory of the game has come about through nearly 30 years of writing and is ever expanding. I read more than I play, and that's the biggest reason that I got into
40k in the first place. Yes, the minis are ok, the rules are fine but there are sharper rule sets out there. It's the whole universe I like, and still do, despite certain writers (cough *Ward* cough) trying to bend or even snap the already established canon to suit his needs.
All in all, I love the rich universe and this is what keeps me coming back to
40k, even after 18 years.