Why Diablo 3 is not as addictive as Diablo 2

A link to a guy giving his “scientific” explanation but the comments are interesting if you have the time to comb through them. Here’s the top rated comment:

Whilst I’m not qualified to comment on the science, I can certainly agree with the conclusion. In Diablo 2, the hunt for loot was part of feeling the “hero” – keep digging for treasure, keep dismembering those skeletons; somewhere, there’s that Epic Item with which you can win the day!
I’ve played Diablo 3 through to “Hell” difficulty, and I’ve not been using my own looted gear since the middle of the “Normal” mode – there’s just no competition between drops, and what you can buy on the AH for very little money. The “sensible” play style is just to farm gold, and buy AH kit. Found an item? Sell it for gold (or, very occasionally, AH) – don’t use it!
It’s hard to feel like a hero when you’re popping off to Macy’s every few hours to grab a new +1 Sword of Wounding. Your quest to save the world – sponsored by Nike?

Response#1:

Another thing, related but not frequently discussed, is the change in how important gear is vs. other factors. For instance, in Diablo II a high level item would give +20 vitality. Your character got 5 stat points per level which you could, if you wanted, put all into vitality. This high level item was roughly equivalent to only 4 character levels (of which you got 99).
In Diablo III, a high-level item is giving you 200+ vitality. I don’t know about other characters, but I believe my Monk is getting 2 vitality per level (and there are 60 levels). A _single_ item can give me more of a stat than I gain from leveling all the way from 1 to 60. As such, leveling up doesn’t do much other than allow equip items with better stats.
Because items in Diablo II gave, comparatively, closer benefits to gaining another level than those in Diablo III, I felt like I was making progress with either a level up or a nice item drop. If my character was getting decimated in Act IV of Nightmare, I could go back and gain a few levels and maybe get some new items in the process. In Diablo III, not only do your levels not increase your survivability or killing power in any way (besides new skill runes), by the time you feel like you aren’t strong enough to defeat an encounter you’ll most likely have few or even no more levels left to gain. The only tangible measure of progress is “phatter lewt” at this point.