The motivation in D3 is finding the gear you want.
The motivation in the other game is not the gear - you'll probably find that Legendary that summons zombie minions and be super well geared within a week, so the motivation lies in the challenge - there is no limit on the difficulty, it just keeps getting harder and harder. Then if you find yourself getting bored/finding it too hard there's random maps with interesting handicaps on them.
Different things motivate different people so I guess both won't appeal to everyone like me
Edited by Sainesk - 9/24/12 at 10:02am