Mainly reiterating what has already been said:
Blizzard polishes the heck out of their games. This polish extends to areas that many other companies don’t seem to spend a lot of time on, like the user interface and tutorial levels. This helps makes their games very easy to start playing. You don’t have to spend minutes or hours just trying to figure out how to play the game. You simply start playing the game.
This level of polish is also evident in how (relatively) un-buggy their products are. Every complex piece of software has bugs, but Blizzard games tend to have significantly fewer bugs than other PC games.
Blizzard games also have very reasonable system requirements. Some games push the envelope on graphics or computational needs. Blizzard chooses instead to make games that can be played even on fairly low-end systems, and they play well on average systems.
On the other hand, I do have some complaints:
I thought Starcraft II: Wings of Liberty was very disappointing. The worst part by far was the so-called “story”. I usually don’t care much about the story of a game, but it was impossible to ignore it in WoL due to the numerous cinematics, cut scenes, (terrible) in-level dialogue, etc. Basically, if I had been more easily able to just ignore the story, I wouldn’t have cared how bad it was. But I couldn’t, and it was bad.
It also seems to take a very long time to load (or reload) a level. I don’t think it ever took Diablo that long to load a level, except maybe on the PlayStation. It seems I could spend a lot of time just waiting to play, so I don’t bother to play anymore. I will probably not buy either of the two expansions for Starcraft II.
I also think that World of Warcraft is a giant convoluted mess of a wanna-be social network in search of a game, but that’s more “not my cup of tea” than a real complaint.
I think my biggest complaint about Blizzard is that they seem to be getting dangerously close to focusing on the polish to the exclusion of the thing they are polishing. That’s rather subjective, though, and all the positive things I listed above are still true.