I can think of no other company whose games have been played more extensively. All of their games thus far have been dominated PC gaming, and the only reason any of them became obsolete was Blizzard made something else to replace them. All of them have had expansions, and all of them have sold amazingly.

RTS line: Warcraft --> Warcraft 2 --> Starcraft (still being played now) --> Starcraft 2
...........................................................--> Warcraft 3 (still being played now) --> Starcraft 2 (but DoTA will still be played for a long, long time)

RPG line: Diablo --> Diablo 2 --> World of Warcraft (still being played now, and will still be played for a long, long time)
.......................................................--> Diablo 3
.......................................................--> New Blizzard MMO

None of these games stopped being played until Blizzard made a replacement. No game was ever able to successfully compete within the same genres (C&C, Baldur's Gate, Warhammer, etc. franchises are far second places).

My question is, what makes Blizzard games so special?

I know they're good, but I daresay Baldur's Gate was technically a better game than Diablo 2. So was Neverwinter Nights. But somehow, all the LAN shops were packed with D2. I thought Command & Conquer Generals was a better game than Warcraft 3. But again, somehow, everyone plays W3 instead.

So maybe it isn't because they're just good? Is there something special that Blizzard does to their games that make them so popular? 

Diablo 2 is still being played a lot.
And I doubt people will stop playing Warcraft 3 once Starcraft 2 hits.

What I want to say is that Blizzard is amazing, they just make their games so awesome.

Their games are very polished and cater to the casual broader audience, that's what makes Blizzard 'so special'.

I still play Warcraft 3 with friends. I used to play Diablo 2, but then I got kinda bored. :o

balance, charisma, perfect track record, highly addictive. Add those, throw a blizzard sauce over it and you got absolute perfection in their games.

C&CG better than WC3? ...

Blizzard games have superb online with buddy lists and customizable games. The units(rts)/heroes(rpg)/races are balanced and if not on release then with some patches later, something c&c never will reach.

Don't forget the 'Lost Vikings', that was a fantastic game and still being played today (albeit by roughly ten people...).

Actually they cater to everyone, having both a highly intuitive single player campaign and a complex multiplayer mode which requires extensive knowledge and practice.

So I would say, their games are very polished and cater to a wide range of demographics.

Something similar can be said about Nintendo. Is it a coincidence that they're two of the most successful companies in gaming? I don't think so.

I agree. I should have put it that way.

blizzard games are all about gameplay. no blizzard game ever set a new standard for graphics, because they know thats not what matters. they play and control very, very polished though.
additionally they don't milk their franchises but instead support their games for many many years. gamers love that! well, as long as it's not an ordinary shooter with a 4 hours campaign and online for as long as the next one releases (about 2 months later...).

so far they never released a game because they had to, they will take their time ("when it's done") or even scrap a game all together (sc: ghost) before they release something they don't feel is above all competitors.
customers are not idiots, they know when they are milked (although that seems to be no problem for them with the flood of guitar hero games....yet).

imo the blizzard games ALL become classics because they always have multiple hundred hours of fun in them.

...i just hope blizzard stays that way, now that they are with activision!

ps:
a c&c better than war3? imo the last c&c that could compete with a blizz rts dates back to the warcraft 2 days when it was still westwood.