How chess has developed..

..over the past thirty or so years. i listened to an interview with kasparov. when he was asked about hypothetically playing fisher, he answered that fisher would need time to play the game as its played now for a while first. saying that chess continues to develop and change. i wished i could ask him how it has changed specifically over the last few decades. what would fisher need to adjust to? can someone here tell me?

Two things come to mind.  The first is obvious: new opening lines are always appearing, and at the GM level it's important to know them.

The second is the computer, which has elevated chess defense to something respectable.  A lot of the positional sacrifices you see in older games are spectacular and inspiring, but also unsound.  In particular, I can think of a few Tal games that don't hold up when plugged into a chess engine (but I admit that Tal is an extreme example).

But OTB, you're facing a person, not a computer. Objective soundness or unsoundness of a particular move isn't as important as the opponent's ability to unravel it all under pressure.  

Defensive technique has probably improved more than attacking technique... and this is an on-going trend that has been gradually developing for nearly a hundred years.