Dull simplicity with better and better graphics is what it amounts to.
For instance take Heroes of Might and Magic 3 Complete. There are 122 different types of units each with their own advantage against other types. Take into accound the building order of your castle, resources, map size, difficulty, artifacts, terrains, etc, and you have an enourmous world with a real challenge to learn it all and test yourself against what you have learned. Each time you play you learn something new that can be applied to your next playthrough. Each map posses a different challenge making no two plays throughs the same.
Diablo 3 has numerous creatures but only a few are distinct and pose a separate challenge to fight against. The real challenge is honestly battling the different combination of elite skills which is not difficult to learn. You need NOT learn the heirarchy of any item type because level requirement and stats are the only important factors, which the best are lvl 63 and the stats can be bought off the AH. Learning different skill combinations was a huge and fun task when the game first came out but honestly I don't know of anyone now who plays that hasn't looked up online which skill combinations work best (opposite of what Jay Wilson said would happen with D3, imagine that). I believe it was said that D3 was made so that anyone could pick it up and play, and when that is the case, it is usually a game that will quickly become boring and seem like a worthless $60. Once you are through the static content, you are through the content, end of story.