Yesterday you said that D2 had its longevity due to trading and D2JSP users. You also stated that D2 was ridiculously easy for those people. If difficulty is directly tied to longevity, then why does D2 have such good longevity?
Because of traders. I specifically stated that the ease of difficulty was bad for traders, but they stuck around anyway because the basic loot/trade system was so good, and were D2's longest term customers.
Maybe the fun isn't tied directly to difficulty, but is instead tied to the gameplay loop being fun, and the drops being rewarding.
It's all of the above, I think. Something which isn't difficult but is fun can be completed very quickly almost by definition, and anything that can be completed quickly obviously won't last as long. But again, trading and PvP may sustain it despite this.
The 100 yard dash is popular not because it's difficult to make it 100 yards, but because it is difficult to make it 100 yards in under 10 seconds. That's the endgame of D2. And it has kept those people that you believe Blizzard designed D3 around hooked for years.
This isn't what kept most people playing, it was mostly trading and PvP. The actual game content itself was almost meaningless precisely because it was so easy. I was hoping that increased difficulty could give the gameplay itself meaning (in addition to AH/RMAH/PvP), but perhaps that won't be the case, we'll see.