Lack of Creativity in Serving
I'm always amazed at how simple most players serve. Serving is the most creative part of the game (though receive is close), and yet most players seem to serve with little purpose or variation.
A major reason for this is because most players play the same players at their club over and over. There are all sorts of little nuances you can do with your serve that can give opponents trouble - last second changes of spin and direction (via last-second changes to the racket's motion), widely varying spins and placements, serving the extremes (deep breaking serves to backhand/short to forehand, or short heavy backspin/short side-topspin), or just different serving motions - but few use them. Many probably experiment, but since they play the same players over and over, opponents quickly get used to them, and the advantage of these little nuances mostly goes away.
Now even in practice there are ways to overcome this. If an opponent adjusts to your variations when serving from the backhand corner, for example, try it from the middle or forehand side - you'll be amazed at how much this changes things. Or just come up with variations. The more you have, the harder it is for an opponent to get used to them all. Or just hold back on certain serves for a while, and then, when you come back to them, they are effective again. Meanwhile, while you use those newly effective serves, hold back on some others for a while. (When I say hold back for a while, I mean both for a few games or for a few weeks of play - both ways work.)