You forget that the true smash hit was Warcraft II. But to say that Warcraft was like Dune 2 is totally forgetting what the two main rivals represented, and by rivals I mean Westwood Studios and Blizzard. So with Dune/Command and Conquer you had the nameless unit, weak soldiers but strong vehicles. In Warcraft, you had badass units that sometimes only the sight of an enemy mage or catapults near a bridge crossing would make you stop.
And the thing about Diablo is that it was a combination of the fantastic music (I still listen to Tristram from time to time), amazing graphics, one of the first Windows 95 games, top-notch story (I mean the hero sacrifices himself in the end, wow) that made it feel so immersive that you can still play today and have fun with it.
And even today that bar is being raised all the time. Multiplayer games in Battlefield are something to behold, World of Warcraft and Guild Wars 2 are a thing of beauty, Bioshock Infinite has got to be the best violent and artsy game ever. Plus you get indie stuff like The Cave, Don't Starve and you suddenly realize that it is really cool to live in this time and age, especially for not having to deal with autoexec.bat and himem/memmaker stuff just to play your goddamned X-Wing.