This is a truly addictive game, folks. I'm talking heroin for the mind here.
I actually avoided Diablo I altogether and put off playing Diablo II as long as possible. I knew that all my friends and many family members were playing it constantly, but I had better things to do. I have played Role Playing games for decades (in fact, I have one of the original Greyhawk/Blackpool sets somewhere in my attic from my D&D days), so it was not any personal distaste for the genre. I just knew there was some reason I had to avoid it. I played dozens of other Computer RPGs and RTS games in the meantime and enjoyed myself thoroughly. Finally, when the Lord of Destruction Expansion patch came out, I could avoid it no further. All the reviews insisted that it was the most wonderful thing ever, and I decided to give it a shot (literally, as I prefer archer characters that can stand away from the big ugly monsters and not get immediately smashed to the ground). Needless to say, I soon realized why I had been avoiding it. I was completely addicted. So much so, that I am STILL playing it to the exclusion of Elder Scrolls, Neverwinter Nights, Heroes of Might and Magic, Wizardry IX etc. I should note that I actually own and hugely enjoy each of those titles- they just can't compete right now.
It's not that the game itself is all that complicated. It's more the wide variety of ways characters can be developed. The huge assortment of set items, unique weapons and apparel is amazing (and frustratingly difficult to collect). I have a personal goal of collecting every item possible- legitimately, not through some script hack as many other players seem to do- before actually stopping. Now that the latest patch is nearly complete, I now have even more things to collect. Somehow, Blizzard struck a near-perfect balance with a simple game that can be rendered as complex as your personality requires.