Replay value is a huge issue as well. If you don't want to play the game over and over, and brag to all your friends about how amazing level 7 is, you're not going to want to tell them to buy it. Graphics come second to gameplay.
Classic Example: Twisted Metal Series
For those of you that don't know, Twisted Metal 1 and 2 were just absolutey amazing. The graphics were standard PS1 graphics, still a little pixelated, not fully taking advantage of what the console could handle, but good enough to tell the difference between an innocent bystander just trying to cross the street, and a Fire missile.
Then came Twisted Metal 3. This game was produced by an entirely different company. Nullsoft (or whoever produced the game) sold the game title, and let these other guys that were more concerned with graphics go and mess up everything the original TM team had worked so hard for.
Same thing with Twisted Metal 4. It wasn't until Twisted Metal Black for PS2 came out, that our faith was restored, however at a heavy cost. Most gamers had lost touch with TM, vowing never to trust another game bearing that name.
It did however kick ass, as the original game creators showed they knew replay value was second to none.
Now let's all just continue to hope that the Blizzard staff can hold true to their audiences' expectations.