Diablo games become like obsessions, and like any obsessive-compulsive knows, slight changes to routine can cause you to lose your shit. That means that you might like or loathe Diablo 3, but if the first game was your first love, you'll likely long to return. The Diablo 1 HD mod makes that much easier by increasing the game's resolution, introducing support for widescreen displays, improving the interface and making it work better on modern copies of Windows.
It also goes a step further - for better or worse, I'm not sure - by adding extra content to the game alongside the graphical upgrade, including Barbarian and Assassin classes, new items and crafting recipes, and new locations, bosses, spells, skills and more. There's a trailer below showing the destructive scale of the thing.
Diablo 3 was my entry to the series, but I know what it's like to lose convenient access to a beloved game. This should make it easier for those who want to return. To play, you'll need the file diabdat.mpq from your Diablo 1 CD, and to download the mod and its small patch from the site. There are full installation instructions inside.
What may disappoint some is that the revival doesn't yet include the Hellfire expansion pack content, and there doesn't seem to be any plans to add it in the near future. Still, there are new quests to explore instead, detailed here.