Like being kicked in the groin by Ralph Macchio and having Mr. Miyagi standing over you laughing.
I have to assume that you play as Daniel Larusso, since there is no story, and there are no cut scenes to be found. On the first stage you fight against 4 karate "champions" that are all the same exact character in a different colored gi. You then move to stage two where you fight 2 ninjas over and over again. We will call the first ninja Dave, and the second ninja who is Dave's identical clone, Daave. When I say you "fight" them, what I mean is they run at you or they fall through the floor onto you. You simply have to press B to kick them, which makes them fall through the bottom of your TV and disappear.
When you can make out an open door in the background you can press "up" to enter it where you will be confronted with an awful, yet simple, mini-game. The mini game you get to play is totally random. You could catch flies with chop sticks, break ice blocks, or dodge a swinging pendulum. These mini games sound almost like fun, but they aren't. On top of that they only last about 10 seconds, and then you are back to fighting Dave and Daave.
Stage 4 is one of the most frustrating parts of video game history. On stage 4 The Karate Kid suddenly becomes a poorly controlled platformer. On top of bad controls there is a typhoon which causes wind to constantly push you backwards. Dave and Daave continuously run at you and fall through floors on top of you. Should Dave and Daave come at you from the left and the right you can be helplessly trapped while they juggle you in the air. This will almost always result in you being beaten to death, or being thrown into a pit, where you die anyway. Rarely you will get lucky and either Dave or Daave will fall into the pit themselves and die.
When you get to the fifth stage you will run through Dave and Daave again until you get to the same boss you fought in the first stage. Should you beat him, you win. Game over.
All of this can take about 20 minutes if you are good enough, and you don't get totally cheated on stage 4. There are no continues, so it would take most people an hour to an hour and a half to master and completely finish.
Graphically The Karate Kid is absolutely horrible. The backgrounds are nothing spectacular, but the character models are just undefined, and terrible, plus there are only 3 of them! There is Daniel, Dave, Daave (who is a clone, remember?), and the "boss".
The song that plays in the background is extremely annoying and repeats itself every 30 seconds or so.
The controls are awful. "B" is kick, "A" is punch, "up" is jump. To do a "special move" you have to stand perfectly still and press your "A" or "B", which makes you almost never even use them correctly.
Knowing what I know now about The Karate Kid, if I didn't already own this game it would be one of the very last NES games that I purchased. I would range this game from $1-$4. For $1 at least you can see for yourself how horrible it really is. For the collector, $4 is about as cheap as it gets for a nice clean copy of the game.