Rackets - they got more forgiving and powerful. I think even Mac now serves faster with a modern graphite racket than he did 30 years ago with a wooden one.
Strings - with the invention of polyester strings which snap back after "gripping" the ball during the stroke, the amount of top spin applied to the ball has gone through the roof. That means players can string their rackets at lower tensions, allowing even more power on their strokes (including the serve).
Overall physical fitness and training standards of pro tennis players - today's top tennis players are simply better athletes.
Poly definitely changed the game in many ways, but not as much when it comes to fast serves. Even strung in the 40lb range, poly strings generate much less power than gut. You'll find that most of the big servers still use gut in the crosses (and they'd probably use all gut if they could just hit serves and no groundstrokes).
The idea is that because of the poly in the mains you can generally string your racket at a lower tension (including the gut in the crosses), which increases power (without the gripping poly you couldn't use low tensions because the ball would fly on you on anything but the first serve).
The advantage of poly isn't that it grips, but that it's elastic enough to deform and snap back quickly. The old idea of string having 'bite' has been mostly disproven with the advent of high speed video to show what's really happening in the fraction of a second during impact. Those geometric extrusion strings are more marketing than anything else (and shred your stringer's fingers).
I play with full poly at 48#, but have experimented with gut hybrids and full gut at times. Even comparing 48# poly to 60# gut, the gut is like a rocket launcher compared to the poly. The amount of power it generates, at any tension, is unreal. You can hit huge flat serves with it, but the lack of spin isn't worth the extra pace.
> How about 60# full gut vs 50# hybrid (gut in the crosses)? Would the former still be a lot more powerful than the latter?
The hybrid setups I tested were usually in the ballpark of 46-48# poly mains with 50-52# gut crosses. My experience was that the hybrid setups were (obviously) more powerful than full poly, but still very noticeably less powerful than full gut at any tension. Without poly's spin/control, the full gut setups were a big disadvantage from the baseline, but I could hit some amazing (for me) first serves.
I didn't experiment too much though because I was breaking every gut hybrid I tried in a few hours, which got expensive quick.
This pretty much covers it. These factors also radically transformed how the game is actually played, too. Mostly gone are the defensive counter-attackers like Michael Chang due to incredible pace that ground strokes are hit; winners are routinely hit from beyond the baseline. Such a concept was rare just 20 years ago when Chang was world rank 2.
For some beautiful writing on the subject, the ever eloquent David Foster Wallace is someone to look up.
From McEnroe's era to now, the biggest difference is the materials used to make the racquets, which allows them to be stiffer, lighter, and have a larger head all at the same time. All three of those things contribute to how much power you can generate on a serve.
I'm sure it's all of those, but also better players with better training. Most of the best athletes in history are probably currently playing in most sports, as a result of better training, better nutrition, and a larger population to select from.
My nephew played in a minor tournament that Sam Groth won a couple of years ago. He said Groth did a second serve kicker that bounced over his opponent and continued over the back fence.
