I suppose I'll give it a go.

It's RPGish enough to capture a portion of the RPG crowd. It's twitchish enough to capture a portion of the twitch-gaming crowd. Basically, you pick a character and run around a big world attacking enemies, looting the dead, and working towards more powerful skills and equipment. You need not invest much time in the story; you need not invest much time in the skills/stats system. Mostly you just click on things. Click to attack. Click to pick-up. Click to move. Click to open. Click to talk. Click to skip the dialog and move on. Of course there's a bit more to it than that, but not much.

It may sound as though I'm being dismissive, I don't mean to be. Not at all. While it's clear I'm not a hard-core diabloite, I've played through D2 more times than the FDA recommends and am anxious to stop writing this post so that I can go play more D3. When taken with a bit of perspective, I think even the most devoted fans must be aware of the simple truth: Blizzard has successfully hacked some reptilian portion of the human brain; Diablo is merely their way of monetizing this R&D.

To clarify:

If you've ever gambled, you'll know the feeling of anxious anticipation that takes over as you watch the wheels spin, the dice tumble, or the cards flip. Basically, what Blizzard has done is take that experience, that moment, and condense it down into it's purest essence. Every time that an enemy's health bar drops to zero, you see a bit of loot come dropping to the ground. There is a sound and a small pause as it falls to the ground; during this time, you don't know what it is. Every time you hear that sound, you have a small but non-zero chance of finding the greatest item you've ever found. And that, my friend, is why we click.

Join us. You will not be disappointed.