The Infamous Goat Puzzle
Looking at the puzzle with ten years of additional experience and hindsight, it's easy to see what it was that caused many players to regard it as such a stumbling block. the puzzle broke the partnership the game had established with the player. For the many players who worked out how to solve the puzzle this may not be obvious, so I'll explain what I mean.
The developer of any game creates a partnership with the player by establishing a set of rules and gameplay features with which the player can expect to work through the game, defeating enemies, solving puzzles, etc. When those rules are broken or added to without informing the player, the partnership is broken.
***SPOILER ALERT***
In the case of the goat puzzle, a feature was included that didn't appear before this point and wasn't even hinted at. The goat was tied up, but it's chain was long enough to prevent the player from reaching the entrance of an underground dig and butted George, the player character, each time it was tried. At the other side of the screen was an old piece of farming machinery, but the goat prevented the player from interacting with this by again butting George, who landed on his back each time.
Because of the nature of the gameplay up to this point, the natural conclusion the player would likely reach was to find an inventory object that would distract the goat in some way, but this wasn't the correct thing to do.
The actual solution involved trying to get to the dig entrance and George being butted by the goat. Then, as George stood up and the goat returned to its original position, the player had to click on the farming machinery. This caused George to jump up and run across to the machinery, move it a little and then wait for the goat to charge. The new position of the machinery caused the goat's chain to become entangled. This meant that George was able to move to the dig entrance quite freely.
The problem was that George had never run up to this point in the game and the player wouldn't necessarily make the connection that clicking on the machinery at a key moment would make this happen. This was also exacerbated by the fact that there had been no timing critical interactions of this nature prior to this point. The game had established a straightforward interface and gameplay style that suddenly was added to without making the player aware of the additions.
The gameplay consistency up to that point was one of the strengths of the game and the partnership between the game and the player was a strong one that suddenly broke.
In order to maintain the partnership in the best possible way throughout a game, a development team should put themselves into the mind of the player throughout development. The player doesn't have the in-depth knowledge of how the game was put together or the detailed design documents and the connections the developer makes are not there for the player.