Blizzard has a mantra for release times for games. "We will ship it when it is ready." They develop games fairly slowly and release dates regularly get pushed back a year or two but when they do finally ship a product it is generally well tested and free of major bugs. Obviously they don't have a perfect product but it is very widely acknowledged that their final products are extremely polished compared to nearly everybody else. Their trick is two things: First, everyone has a tremendous expectation of their products so shipping something that isn't quite done and patching it up later would be a colossal disaster from a public relations standpoint so they have a huge incentive to never do that. Secondly they are the big name and have the cash and clout to push back release dates if they feel they need to, which most other game companies do not have. Many companies have to ship products that they know aren't done properly and they desperately patch them in the following months to get things right because they are pressured financially or ordered by the companies that own them to ship on a particular date to make the numbers look good.
The easiest examples are Starcraft 2 by Blizzard and CiV by 2K. SC2 was very polished when it shipped, was free of major bugs and the balance of the game was excellent. It was widely agreed that the product was extremely well done and it was well received, barring the usual internet hatred of everything. There have been very small balance tweaks and bug patches in the interim but I would be very happy with playing the game just as it shipped. CiV was not this way at all; there were large numbers of really substantial bugs and the game balance is off in a lot of startling ways. The game crashed semi regularly and often games had to be entirely abandoned because they would always crash on a particular date. The AI was extremely easy to defeat and had lots of glaring problems with its behaviour. The game was still a ton of fun of course but it was clear to everyone that the game was rushed to meet a ship date instead of shipped when it was done. It is certainly frustrating to be a gamer buying a much anticipated game that looks amazing and then come to understand that it will be necessary to wait a couple months for all the patches to get things really working properly. Having played a few games that had some very serious bugs at ship (*cough* Temple of Elemental Evil *cough*) that were never ever patched I can certainly appreciate the company continuing to push out fixes to make things better.
I must give credit where it is due though, CiV has been substantially fixed in the past month. The AI has been drastically improved in numerous ways, so much so that I think playing on difficulty 6 at launch date was about the same difficulty as 4 or 5 now. Many critical bugs have been squashed and things are looking much better, though there remains some number rebalancing to do. I do wonder why the high ups that run game companies so often push out products that simply aren't ready. There are good reasons why I will buy any Blizzard game as soon as it comes out and the primary one is simply that they are always *finished*. Pushing out a game early and getting some cash in the current quarter is fine and all but you lose out on lots of sales due to negative reviews and you lose out on customer retention when the game doesn't live up to the hype and promises. It seems like there are a lot of people out there who make the final decisions about games who refuse to take a long term view of things and it costs them a lot of money to do so.