In addition to pointing out that lacks the hard visual edge of previous games in the series, Roper reveals that he likes the darker presentation of the Diablo universe featured in classic Diablo.

You know, I liked the darker grittier. I liked the differences in art style, to be honest. So, I think I would personally from a player standpoint prefer that. -Bill Roper

He also goes on to explain why he thinks there's a change in art direction with Diablo III.

One of the things I always enjoyed about that separation between Blizzard and Blizzard North was that the Diablo games had a very distinct art style. They had different art directors, they had different people working on it, they had a different sensibility about them. Diablo was I think grittier and darker and a little more leaning towards the photo realistic. Whereas the Craft games that were being built down in Irvine were bigger and broader in scope, brighter colours, just different pallets and different presentation.

Finally, Bill Roper clarifies that even though he prefers the more ominous look of classic Diablo, he doesn't hate how Diablo III looks.