Maybe this "Flaw" only serves to show what kind of replay value a blizzard RPG has. The game content is not lacking, the formula is. When a game's contest is in how long you've been playing (As it is with an MMO, whether measured in levels, or Gear, or achievements) you end up with people just blowing their time away staring at the screen, having the minimal amount of enjoyment neccesary to keep them playing. Every game gets old and passe after time, but Diablo is way lower on that spectrum. When you get to the end, you've already heard the story (Which in Diablo 3 was so predictable, you knew it before you even finished it), you've got no more goals (You already have all the gear you need to overcome any challenge), and it wasn't really a thrill to begin with, so you're left apathetically wandering off to something else. I could list the problems out, but I don't want Blizzard to gather them as data, and just use it to tweak their one game story formula.
I have a thought here, maybe it is a stretch here, but bear with me, it makes sense. Most people working a job, be they CEO or janitor, want to try to keep earning more, while doing less work. That's human nature. Of course, this causes the quality to go down. A CEO takes a huge risk by running a company, so to do less work, they try to minimize the risk. Nothing about this makes someone a bad person, but to minimize your risk in the game industry, you analyze demographic charts, polls and come up with a formula to make the game that will make you the most money. Well, This is the result, your formula was designed from charts and graphs, and it dominates that world, but you failed to make a game with lasting replay value. If you are a CEO whose job it is to set the direction of the company and take risks, then do it. Your product will not be as good unless it is Do-or-die, Sink-or-swim, Eat-or-starve. Take risks on new ideas, and make a game designed to express your artists' vision and please your players, not with the sole goal of making money. Blizzard could learn a lesson from Valve in this regard.