A1: The Telecommunications Act of 1996.
This is probably the most often ignored part of the bubble in my opinion. Deregulation of local phone service opened up the monopolies to outside competition (which was actually a good thing). Capital flowed into new regional bell operating companies (RBOCs) who over-built fiber optic networks. This fueled the idea that internet commerce was the "wave of the future," and it was only a matter of time before all traditional bricks and mortar business would close. In some sense, the market actually got the story right, but its mistake was assuming that the monumental change would come much quicker than reality.
The dot-com bubble was actually more of a tech, telecom, and media bubble. Most other sectors of the economy were not really impacted all that much. In fact, by the end of 1999, the technology and telecom sectors alone comprised 50% of the combined weight of the S&P 500 index. Today, those sectors are only around 20% of the index.
A2: Corporate abandonment of defined-benefit pension plans in favor of 401(k) plans.
The 401(k) was first available in 1981, and by the early 1990s, companies started to adopt this new retirement program on a broad scale, and in the process, they quickly began to abandon their old defined-benefit pension plans. Thus, the burden of retirement investing shifted from the company, to individual investors. For many households, this was the first time they had thought about the stock market, which became more popular thanks to accessible business media such as CNBC. According to the Investment Company Institute, in 1980, only 5% of households owned an equity mutual fund. By 1990, this figure was 25%, and by 2000, it reached over 50%. So US equity markets saw a massive inflow of capital, which lowered the cost of capital and made it possible for many marginal tech/telecom companies to go public.
Technology also lowered transaction costs associated with equity trading, and discount brokers became more accessible to the average investor. Online brokers began to show up in the mid-1990s, which turned equity investing into a video game. As the markets rose, day traders proliferated. By the end of the "bubble" many institutional and individual investors were investing on fear. Fear that they would not be in the market, and it would continue to rise.
Bubbles are actually very common in markets throughout history, so the dot-com era was nothing new. The severity of the correction was certainly larger, but there are many examples of similar bubbles in different industries: tulip bulbs in the late 1600s, Railroads in the late 1800s, autos in the early 1900s, oil drilling in the 1970s, LBOs in the 1980s, tech/telecom/media in the late 1990s, real estate in the 2000s, gold currently.