Predictability:
Hard, flat hitters need predictability because unexpected racquet angles are the last thing you want when you're trying to hit through a narrow margin above the net.
A racquet that twists and turns is just as problematic, if not more so, for a player who hits heavy topspin, because if you're swinging upward to create spin and the racquet turns or twists upward, the upward tilt not only sends the ball on a higher trajectory, but it also reduces the brushing action through which the strings give the ball topspin. With more lift and less topspin, your ball will go a lot farther than you intended. Furthermore, if you're trying to hit topspin, you can't avoid unexpected tilts by perfecting your skill at meeting the ball exactly on the long axis. Topspin strokes generally require the ball to impact the string bed above the long axis, roll downward across the long axis, and leave from a point below the long axis. For topspin players, variations in up-down racquet tilt have a major effect on depth of shot, and a lot of topspin players address this problem by leaving a huge margin for error. They end up hitting at an average depth closer to the service line than the baseline, but they would be much more formidable if they could safely aim deeper.
In terms of the racquet frame itself, stiffness enhances predictability. So does greater string tension, because the ball leaves tighter strings sooner, thus giving you less time to accidentally change the angle of the stringbed while the ball's still there. If you string too tightly on a stiff frame, though, your arm will feel the effects of the condensed duration of the ball-impact shock. This imposes an important limit on how far you can go in seeking predictability.
A limited power ratio:
Each player has a limit on the degree of upward cut (to create topspin) that's feasible per average swing, so if your racquet has too much power for the amount of spin you can generate with a full-speed swing, you'll hit long. The more recent laboratory studies indicate that there's not a great deal of difference between racquets and strings in terms of how much spin the ball receives, given the same swing path and speed. Tighter strings, rougher string textures, and wider string spacing help, but not by a factor of more than 10%, and racquet frames themselves determine spin even less. The key, therefore, to finding the right power-to-spin ratio is the racquet's power, not its spin potential.
For the flat hitter, the analogue to the power-to-spin ratio is the power-to-accuracy ratio. At a given swing speed, a more powerful racquet will require a flat hitter to aim through a smaller margin above the net. If you gain at least as much predictability as power, though, aiming though a smaller margin won't be more difficult, because a more predictable racquet gives you better control over the height of your shot.
Mass you can maneuver:
Given all we've said about the disadvantages of lighter racquets, we should note their virtue: They're easier to swing fast and to get in position for quick reactions. You don't want a racquet that's so heavy, you feel encumbered by its weight, but for adult players of average strength, such overweight racquets are quite rare in the current market.
Arm safety:
Racquet weight and stiffness can make a huge difference to the health of your arm. When a light racquet head turns in response to an off-long-axis ball impact, the turning force (torsion) is transmitted through the racquet handle to your arm. A light racquet also absorbs less of the basic shock of the ball's impact, whether you hit on the long axis or not. Torsion and shock both commonly lead to tennis elbow and other injuries. In one sense, a more flexible frame mitigates these problems by spreading the torsion or shock over a greater period of time and thus reducing the peak strain on the arm, but a more flexible frame also vibrates back and forth more violently after impact. Although no one can see this "flutter" with the naked eye, many players who aren't used to flexible frames can feel it quite distinctly. Flutter has not been proven to cause injury, but to players who notice it, the discomfort it causes to the arm is much more concerning than the purely aesthetic annoyance that string vibration causes to the ears.
So, what's the ideal racquet?
