Art design is much more important than level design in a game like Diablo. The action was so simple that level design would have made no difference, so long as it looks the same.
Random levels only helped it (immensely) by giving it enormous replay value. You had an idea what was around the next corner, but you were never certain, and had to explore everywhere to find those chests. If it wasn't random, you'd always beeline the same place.
It's like X-COM or Civilization. Would level design have helped them more than random levels? Not at all.
And what are your feelings on randomly placed enemies? Jagged Alliance 2 had predesigned levels, and it hurt the game's replayability a bit, but it compensated by completely randomizing where the enemies would be, right down to whether or not a map would have any.
Some games benefit much more from randomness than true design.This comment was edited on Apr 27, 10:08.