How have tennis racquets changed over time?

It depends. In general, a well balanced and well constructed racket will enhance your movement and control of the shuttle, however, many high-end rackets are heavier and very stiff which makes them difficult to use unless if you have a strong arm and a well developed stroke. In return they usually provide more power and control if used correctly. The racket string also helps you to play better by providing more power and control. Many of the stock racket strings are too loose to be any good, in general most people string between 20lb and 24lb.