Lots of good points made above...
I think beginner and tweener frames have gotten much better - more stable and more comfortable. The heavier frames didn't really have these issues to begin with.
In beginner and tweener frames, there's been a huge downward trend in weight and, along with that, significant increases in stiffness and head-heaviness (they had to -- to get more power after reducing the weight). And they have indeed developed some technologies and designs that have increased stability and comfort in these frames.
Some would question the merits these very lightweight, stiff racquets. Yes, they allow beginners to "get going" more easily, but perhaps at the cost of developing bad habits (such as short swings, wristiness, etc.) and, in many cases, at the cost of tennis elbow and arm and shoulder problems. It turns out that heavier, more flexible, more head-light frames
are better in the long run.
Even player's racquets have gotten lighter -- strung weight of the Speedport Tour is 11.7 ozs, way down from the 12-13 oz range for player's frames from the late '80s and '90s.
The strings are better, non-gut strings are used by most professionals now. Even multis of high quality are used in placed of gut.
There is more selection to racquets now and marketing non-sense, the racquets themselves seem to be made of cheaper filler material which gives them a dead feel with the exception of a handful of racquets.
I find there is less emphasis on control and more on power and spin generation nowadays. Raquet head sizes have gotten alot bigger and string patterns alot more open. But cheaper craftsmanship and quality overall. I love my dunlop 200g
The game has changed from flatter shots to bigtime power and topspin. I think of the Bab Pure Drive as a nexus racquet in this evolution (although even Fed with a PS 85 hit some heavy-topspin passing shot winners when he beat Sampras at Wimbledon in 2001).
As for the cheaper materials mentioned by Deuce and others, I've been thinking this recently: with frames being much lighter and at the same time much bigger (wider beams, larger heads), the materials in modern frames just have to be less dense, don't they? Along with the increased stiffness, it stands to reason that they're going to feel hollow and cheap compared with the older frames, doesn't it? ...Even if the quality of the materials is the same...
Bottom line for me is I've been really enjoying hitting with racquets from the late '80s and '90s. They're solid, well-made, and they play great for me.