When I scan 15 years of The Forbes Four Hundred, the most powerful impression I get is one of change, of cinema more than portrait. There is the Andy Warhol-like quality to the lists, the incredible churning of the roster as names come and go, reflecting the tremendous impact that technology is having upon the economy.

The sources of wealth have changed: Property and heavy industry give way to intangibles such as computer software and communications. Reflecting increased leisure and disposable income, purveyors of diversion and entertainment have climbed higher on the list.

What is this "wealth" that Forbes so famously measures? We measure wealth by asset values, defined as what people are willing to pay today to own claims on future income streams. This is true whether the future income be royalties, coupons on bonds, dividends on stocks or rents on properties. But not all income streams are priced the same. Growing income streams are worth more than static ones, and reliable income streams are worth more than uncertain ones. In short, not all income streams are equally valuable, but all become more valuable when tax rates fall—as capital gains taxes now have—because more of the income stream remains with the owner.

The largest asset class in the U.S. is land and real property, worth more than $10 trillion today and making up more than half our net worth. Capital goods—tools and factories—and aging consumer durable goods make up the other half. Twenty years ago, in the late 1970s, the prices of all these tangible assets were inflating at more than 10% per year, and were expected to continue to do so. That almost-guaranteed return attracted investors to hard assets and away from stocks and bonds. It made physical properties and resources in the ground more valuable than paper assets, which did not necessarily benefit from inflation and might be harmed by it.

Since then, inflation has declined and made paper assets more attractive. Neither Bill Gates nor Warren Buffett nor Rupert Murdoch owns much in the way of physical assets, which is why people like them have moved up so much on The Forbes Four Hundred list.

Their unprecedented success in building new companies attracted capital to new ventures and entrepreneurs. Think of many of the new rich as income-stream prospectors: people with the energy, vision and courage to build new companies.

The shift from effort to entertainment is understandable if disturbing. Technology has enabled entertainment entrepreneurs to enjoy a dramatic reduction in the cost of reaching their customers at the same time that globalization has helped them build market overseas. Their markets have grown as people have more leisure and more spare cash. This trend worries me. History has not been kind to societies obsessed with diversion, such as the Roman Empire in its late stages.

Another worrisome trend visible in The Four Hundreds is Sudden Wealth Syndrome, the trend toward increasing volatility in asset markets. Fortunes made quickly can disappear quickly, too. In these conditions, hubris is dangerous. A bit of humility can come in handy here.

On the bright side is today's balanced, growing economy. Nearly every region of the U.S. is growing; skilled and energetic workers are in great demand; distressed properties have disappeared; prices are stable; interest rates remain low; and capital is plentiful. Our best brains are being drawn into areas of new discoveries and emerging technologies.

I especially admire those who are using new technologies to radically lower the cost of educating young people via computers and the Internet. I suspect future Forbes Four Hundreds will be replete with names of people who created income streams by making education more efficient and effective.