Difficulty

Balancing is a part of game design I've never been terribly good at.  Difficulty is the core result of balancing.  Difficulty is how easy or hard it is to achieve a winning situation in a game at any point.  In video games, this tends to adjust the number of enemies, health levels, amount of ammo/resources available to the player, deadliness of enemies (how much damage they do and how often they hit successfully), etc.  In board games, this tends to affect how possible it is to come back an win from an underdog situation.  On the one hand, you don't want players to ever feel like they can't win.  On the other hand, you don't want a player in the lead to feel like taking another player down a notch is completely meaningless.  Poorly balanced games can suffer from either being too easy or too hard and from what I'm calling "rubberbanding".

Too Easy

It may seem like it would be fun to play a game that takes no skill to win.  And for a little bit, it actually can be a nice little diversion, but if a game never becomes a challenge at all, then it will get very boring.  At some point, the player will realize they aren't really needed to play the game, if something else pressed the inputs or moved the pieces, the game would play itself.  Some examples of this are Candy Land (is there ANY thought or choice in that game?) , Chutes and Ladders, and most Kirby games (the difficulty is when you try to find all the game's secrets, otherwise the game is painfully easy).

Too Hard

When a game becomes too hard, the player feels they cannot win.  Sometimes they know this going in, in which case it's okay (arcade games or games with high scores tend to be this way).  If they reach this point too often, however, the player will start to feel incompetent and get very frustrated at the game.  This is generally when controllers end up inside of televisions or smashed to a thousand pieces on the ground.  There does seem to be a niche for games intentionally built this way.  Personally, I can't stand games that frustrating.  One tricky thing I've noticed is trying to distinguish between a game being super difficult and me just not getting used to the controls/game actions.  If I'm stuck due to the latter, that means I can get better and overcome the fight, which gives me the rewarding feeling that designers want players to feel when playing their games.

I think an ideal game has a mix of areas that border on Too Easy and Too Hard so that you can have a roller coaster ride effect while playing the game.  For example, immediately after a tough boss battle, you should get some piece of equipment that makes taking down normal enemies a piece of cake.

Rubberbanding

Rubberbanding is when a player in dead last can very easily recover and win the game despite any setbacks that have occurred previously.  Although, this feels great for that player, it makes the rest of the players feel like everything they did in the game was pointless.  The most frequent instance of this is in the Mario Kart series.  I've always felt the computer cheats in that game and recovers too easily, but it's gotten more and more ridiculous as new Mario Kart games come out.  Especially since the accursed blue shell made its appearance.  Carcassone can feel like it has this problem, too, if someone's strategy is putting Meeples on all the big fields since it looks like they have no points until the end of the game.