I really think Diablo II deserves at least a graphics update. But from a rational standpoint, it doesn't make sense to spend resources with this game anymore, since it has no way to provide additional revenue (apart from sales).
Although I own Diablo III and the expansion, I feel very discouraged after playing them for a while. There are no epic runewords to be made or epic TC87 to collect.
It's also funny because I feel Blizzard is desperately trying to appease to the more hardcore players (certainly they're the ones who complain the most). They came up with the bounties, rifts, paragon, etc. Now they want to introduce Ladder and Leaderboards. All end game stuff.
However, with Diablo 2 I don't need Blizzard to keep me motivated. The game itself keeps me motivated. And I think this the major difference between Diablo 2 and 3. On D2 i have so much to look forward to after I complete the game on Hell. So many builds to try on so many classes with a vast array of great items.
I really can't seem to replicate that experience on Diablo 3. And that's a big let down, at least for me.