Blizzard games strong mechanics. They understand what makes a game addictive and worth playing. I can tell you prior to WoW coming out MMORPG was not mainstream, at least not in my community and amongst my friends, after it came out, everyone I knew was playing WoW, and these weren't gamers, they were people that up to then hadn't touched a computer game, or were "I played this funny game where you shoot people, I think its called Quake" kind of people. In otherwards Blizzard took people that largely were not gamers and hooked then onto WoW. They drew new people into the market. That's ultimately what made them successful I think, when they saw Diablo they knew it was something that would appeal to people and ended up buying Condor.
Secondly Blizzard games has strong themes and storylines, this Blizzard owns a huge debt to Games Workshop, whom I am surprised never sued them for copyright enfringment. The green skinned orcs, Zergs, and Terrans all have origins in Warhammer Fantasy and 40k. But not only this they take time to develop storylines and populate their games with memorable characters and interesting plot lines.
Blizzard pays an enormous amount of attention to detail. You can tell when the last half of almost all Blizzard manuals are just fluff, background setting, item descriptions, and story.
And lastly Blizzard games attract a broad spectrum of people. They attract young people as young as 12-13, at the same time the gameplay and content is mature enough to appeal to teenagers and adults alike. Even hardcore gamers like me, feel that Diablo 2 has the occasional hack 'n slash appeal. Compare this to games like the CRPG classics, Baldur's Gate or Torment, as popular as Baldur's Gate 2 was it can't compare to the huge popularity of Diablo, and nothing needs to be said of Torment.
Oh yea one more thing. Warhammer 40k is still 1000x cooler then Starcraft. Dawn of War was the first time I started skirmish games against the computers just to listen to to the CSM go "CHAAAAAAAAAAAAAAOOOOOOOOOOOSSSSSSSSSSSs"