When I coach someone, the very first thing I look at are their grip and feet positioning. If either of these are done improperly, everything in between will likely be twisted like a rubber band into an awkward stroke that players and coaches will often try to fix by treating the symptoms rather than the root of the problem. Get them right, and everything in between tends to straighten out. New players should focus on this to develop their strokes properly; more experienced players should focus on this to correct fix improper strokes they may have developed.
For example, a shakehander might hold the racket with a forehand grip, so that the racket is rotated to the left (for a righty), with the index finger partly off the racket rather than flat across it near the base. This means the racket is aiming slightly downward on the forehand side when the arm is aiming forward. Or a player might use a backhand grip, so that the racket is rotated to the right, with the thumb almost flat on the racket rather than the thumbnail roughly perpendicular to the surface. Now the racket is aiming slightly upward on the forehand side when the arm is aiming forward. In both cases the player has to adjust his stroke to compensate for the fact that his racket and arm aren't pointing in the same direction, leading to awkward positioning of the elbow and shoulder, and even the body and feet can be thrown into awkward contortions to compensate. At the advanced levels, some players do use slight forehand or backhand grips, but only after the shots are ingrained, and always for a specific purpose (i.e. to strengthen one side, even at the cost of the other side). Until the strokes are ingrained, players should use a more neutral grip, so that the thinnest part of the wrist lines up with the racket.
Common foot placement problems include feet too close together when they should be more than shoulder width apart (especially with taller players); feet parallel instead of the tips pointing a bit outwards; weight on the heels or evenly distributed when it should be on the front inside part of the foot; and (for righties) the right foot either parallel to the left, or too far behind it, when (for beginners) it should be in between these two extremes, and so only slightly back. Each of these problems leads to technique problems where the root of the problem often isn't obvious, and so players and coaches try to fix the symptoms rather realize the problems stems from the feet positioning.
For example, if a right-handed player has his right foot too far back, then he won't be able to rotate at the hips, waist, and shoulders, since to do so would mean turning almost straight backwards, and he ends up with an awkward mostly-arm stroke. Seeing this, many players and coaches tell the player to rotate the hips, waist, and shoulders, thinking that's the problem when that's just a symptom of the problem, and so they make the problem even worse. Or if the right foot is parallel to the left foot, it becomes awkward to rotate properly because the amount of rotation needed to compensate for the foot positioning is too much. Again, a player or coach might tell the player to do more rotation rather than addressing the root of the problem. (At the more advanced levels, where players have learned to rotate into shots properly, a supple player often does play forehands with the feet parallel, but that's only after the proper stroke has been ingrained.)
Imagine your body is a rubber band pulled tight. If you twist either side, the rest of it twists, leading to twisted strokes that'll handicap you for the rest of your playing days. Untwist the ends, and everything in between smooths out. Do you want a smooth game or one that is twisted into knots?