A game now outdated to the same degree that it was once ahead of its time: hugely.
Look at the tagline for Diablo's main entry on Gamespot: "Diablo is the best game to come out in the past year, and you should own a copy. Period." And a truer statement was never made...in 1997.
In mid-2009, time has not been kind to the original Diablo. Instead, the usual amount of post-release patches and pampering from Mama Blizzard has long since been showered on Diablo's predecessors: Diablo II and it's expansion, Lord of Destruction. Diablo II at its base level eclipses the original in every possible way while retaining the exact play and game mechanics.
For example, in Diablo, the gameplay consists mostly of what is referred to now as grinding. And yet there's not even a "run" option for your character, so exploring and spelunking includes a ridiculous amount of tedious walking which will be frustrating for anyone who has played any of the modern games that Diablo influenced from WOW to FATE.
Those descendants, particularly Diablo II, fixed this and other obvious omissions to the gameplay, GUI issues, and graphical shortcomings. If you're at all interested in this game, Diablo II (and soon Diablo III) will be your ultimate destination. So much so that it's hard to even google for information about the original Diablo without hitting sites devoted to the sequel. Hell, you can't even buy the original Diablo retail save for getting it as a bonus in a Diablo II bundle.
In context though, when Diablo was released, your next best option for playing Dungeons & Dragons electronically was Gauntlet. Diablo was a huge leap forward and is worth playing if only to see that step being taken and/or to get the background story for Diablo II.