Psyringe: At the moment I'm quite enjoying Titan Quest, which basically is a 1:1 clone of Diablo 2 in a mythological Greek setting. The things I heard about Diablo 3 didn't sound too thrilling, and the "always online required" DRM abomination makes it easy to skip it. Perhaps I'll have a look if they release a DRM-free version in a couple of years.
Not only that, but all Diablo games have massive server problems on launch. They always under-order bandwidth, every game I've ever played, and the game can be barely playable for a week, barely better for the first few weeks, and unreliable for months. That alone is reason to, at the very least, wait out the purchase for a while. The other reason is that every new release has lots of bugs these days, and Blizzard has not been the exception for many years now. Take those two things together, and I expect playing Diablo 3 will be a bear for weeks, even if you love DRM and want to make it your woman.
hedwards: Honestly, I wish Blizzard would be friendlier with mod developers as IMHO that's a huge selling point for a game. Well, the mods that is, you can't assume that because there are tools that they'll become popular.
Druidshinobi: It just goes to show Blizzard or more so Activision just want complete control of their game so obviously down the line they can nickel and dime there customers with overpriced pets or other crap, sort of like what they are doing with world of warcraft at the moment.
With mods it may encourage people to actually buy their game who.. otherwise may not have bought said game. I know i certainly would buy Diablo 3 if there was a MLP mod that changed the character classes to the mane 6.
Expanded storage space. Always comes in the first expansion to this sort of game these days. If they let you use an ATMA-like utility and play offline, it would arguably (thought probably not really) cut into their profits, as having a much-too-small inventory space/backpack is a major motivation to buy a sequel, making it almost mandatory.