I'm noticing a disturbing trend lately, with games being changed through online patches due to something that the developers didn't intend, or developers altering intended features due to a change of opinion.
The problem is that intentions and current opinions don't matter with any other product. You pay your money, you get what you purchased, and you can count on it staying that way. If I own Return of the Jedi on laser disc, George Lucas isn't going to sneak into my house, break my disc, and replace it with a new ending that goes along with his prequels. But increasingly, that's not how the game industry works anymore. Lately, this can even be true for single player / co-op games.
Perhaps the best example of the worst changes I can think of belongs to Battlefield 3. Aside from the absolute mountain of balance adjustments made to weapons, vehicles, equipment and just about any other aspect of gameplay, there were two huge changes that personally ruined the game for me, and basically made me swear off the series.
When the game was first released, the enemy spotting feature could have been argued to be a little too powerful. You could hammer on the "mark enemy" button throughout the game--which would put enemies on every player's mini-map--and it would end up marking enemies who you couldn't even see yet, or who even might be hiding in bushes. If one team was marking enemies and another wasn't, the first team would have a huge advantage.
I didn't mind this, but I guess the developers did, because three or four months after release, they made a rather large adjustment to the feature. They broke it. They reduced the range, but in doing so they somehow also destroyed the reliability of the feature. Many times, there would be an enemy 10 feet in front of me with nothing blocking my view, I would hammer on the mark enemy button, and nothing would happen. They broke the feature, and even after several updates later, they never bothered to fix it. I quit playing several months later--soon after the first DLC expansion--and it still hadn't been fixed. I'm not even sure if it's fixed now.
Then came the private console servers. When I bought the game, everyone used the publisher's servers. But after several months, these previously reliable servers had been rented out to people who wanted to play one map over and over with a ridiculously inflated score limit, that often turned simple matches into grueling affairs that could last for hours. This went on for weeks, and even after it improved, it was still never as easy again to find a standard game with default settings.
I could name other games and other examples, but I'd like to hear about some of your experiences. Which post-release game changes upset you most?