I really tried narrowing this down, but this could just as easily have been a Top 50 list. As with past lists, these are not necessarily in order of importance.

1. Bad technique.
2. Deep down they don’t believe they can play at a higher level and so continue to only play shots at their current level.
3. They keep practicing the same things rather than work on what’s keeping them from improving.
4. They don’t use Saturation Training to fix weaknesses.
5. They don’t know The Book on Your Game.
6. They don’t develop good third-ball attack serves to improve their attack.
7. They don’t make it a habit to serve and attack.
8. They don’t work with a coach and practice partner regularly.
9. They focus on power instead of consistency and placement.
10. They don’t develop their game tactically (what to do in a given match) and strategically (developing your game for the future).
11. They don’t develop an overpowering strength that opponents will fear.
12. They don’t make moving their feet a habit.
13. They don’t analyze their wins and losses to see what works and what doesn’t.
14. They don’t follow the Seven Habits of Highly Effective Table Tennis Players.
15. They aren’t perfectionists.
16. They don’t know How to Move Up a Level. 

1. Playing too much into the backhand.
2. Not attacking the middle (roughly the playing elbow, midway between forehand and backhand), and using this to set up a follow-up attack to the corners or middle again.
3. Not attacking all three spots – wide corners and middle.
4. Attacking to a corner when you have an angle to attack outside the corner.
5. Looking for a chance to counter-attack to an opponent’s forehand rather than just making the first attack there.
6. Not changing the pace.
7. Not sometimes aiming one way, and at the last second going another.
8. Not keeping the ball deep (unless going for a change-of-pace or drop shot).
9. Trying to end the point too quickly.
10. Not ending the point when the shot is there.
11. Not finishing a shot by following through into position for the next shot.
12. Attacking an opponent’s weak side first rather than first attacking their stronger side so that you can follow by attacking their weaker side while also having to move. 

1. Too predictable. Vary your receive! Most of the below are variations on this.
2. Giving the server what he wants.
3. Too aggressive. If you attack every serve it becomes predictable.
4. Too passive. If you never attack the serve, then the server knows he can serve and attack every time. (If he’s smart, he’ll vary his attack.)
5. Passive against deep serves. You can’t really rush or angle the server against a deep serve, so you need to be aggressive.
6. No variation to pushes. Need to vary them to throw off the server.
7. No short push variation. Opponent knows your return will go long and so can hang back.
8. Long pushes aren’t quick, low, heavy, deep, and at wide angles.
9. No last-second changes of direction. So server can see where you are returning the ball very early.
10. Unable to backhand banana flip against short serves, the biggest development in technique in recent times – get with the program!!! (Here’s my Tip of the Week, Backhand Banana Flip.)

1. Not enough variation.
2. Serving to the backhand over and Over and OVER.
3. Always serving from the same spot, usually from the backhand corner.
4. Not serving with a purpose.
5. Always serving with heavy spin, without using no-spin as a variation (where you fake heavy spin – “heavy no-spin”).
6. Telegraphing your serves by how you set up or start your motion.
7. Too often serving long to loopers or short to quick players.
8. Not trying out all your main serves early on to find out what works.
9. Saving your trickiest serves for when it’s close, instead of using them early so it doesn’t get close, and then using them again as needed.
10. Not understanding the basic concept that you should essentially ALWAYS serve and attack UNLESS the receiver does something to stop you from doing so.  

Many players go through this series of events as the opponent is hitting the ball toward them:

1. Opponent begins forward swing.
2. I wait to see where the ball is going.
3. Opponent hits ball.
4. The ball is coming toward me.
5. Do I have to move?
6. Yes, I have to move!
7. I prepare to move.
8. I move.

More advanced players go through this series of events:

1. I prepare to move.
2. Opponent begins forward swing.
3. From forward swing I see where the ball is going.
4. I move.

Which do you think is more efficient? The two most important things from this are that you should get into the habit of trying to react to where the opponent is hitting the ball as he begins his forward swing, and that you should assume you have to move.