I used to play Everquest...despite the name, the game had very few quests. Instead, there was this mindless grinding of monsters to level up in it. After awhile I sorta enjoyed that experience. You are interacting and at the same time, you aren't trying to maximize a rotation or anything. It was a relaxing experience really. When I started playing Diablo 2 many years ago, I enjoyed the thrill of discovery of new items and the same simple low attention, low stress enjoyment from killing. One of my friends would make up stupid goals for himself, like punching every monster in every act of Diablo 2 to death. (He used a bunch of charms for elemental damage. Incidentally, he leveled a Warlock to 40 on a pvp server with no gear equipped. I wouldn't say he was wrong to do such things as you are implying, he just was focused on different things than character improvement. After all, RPGs are Role playing games. On a table top game, we'd call you a roll player rather than a role player.)
With Diablo 3, alot of the "improvement" for me is trying different specs and different combination of runes and abilities. Yeah, I like seeing my dps number or defenses do different things but at the same time, I also like the idea of a budget character. (I used to build 15 dollar or less decks in magic the gathering for fun). There's this beauty in efficiency to me. Yeah, I could spent 12 million (peanuts to most) for a better weapon, but this 2 million gold weapon is almost as good.
My biggest sale was a 200 plus Int ring with a ton of great stats (I think it was damage, attack speed, magic item find, vitality and crit strike). I could have kept it for the extra 10,000 dps or whatever it added, but I figured the 200 dollars was worth more to me than the pixels of the ring. (Plus a wise move in retrospect with all the legendaries and hellfire rings and paragon levels that made the magic find on it worth less than before). In some ways, I guess getting rid of a godly item is a godsent to me because it means the gear hunt can go on. It was more about the journey of grinding than actually getting the gear, if that makes sense. Once you have perfect gear...that's usually when you stop playing lol.