I know some of you are clicking for Wikipedia right now, muttering to yourselves "Seventeenth? Is that income tax, or when they let y'all chicks vote? No, it's Prohibition, right?" No, the Seventeenth Amendment is the one that calls for direct election of Senators. What's so bad about that, you ask? (Go on, ask.) Well, let me tell you...
Understand this first: I'm not one of those people who think the Constitution is divinely inspired and the perfect governmental document. However, for setting up a limited federal government with strictly enumerated powers rigidly separated among different branches that acted as brakes on each other, it's really pretty clever. There was a lot of thought put into a careful system of counterweights and oversights, and if you change one bit of it, you can throw something completely unexpected out of kilter. And boy howdy, did the Seventeenth Amendment ever do that in spades.
Firstly, our bicameral legislature originally copied one redeeming feature from the English Parliament. The lower house was composed of representatives directly elected, one per every X number of the population. Because of their (relatively) small constituencies and their two-year terms, representatives had to be very aware of the popular sentiment of "John Q. Public" and respond to it, lest they be replaced. In the English system, the upper house was the House of Lords, with noble members who had a lifetime tenure. Although they couldn't permanently shoot down legislation, they could apply a temporary veto which could be overturned by a determined lower house.
Our upper house was the Senate, whose members served terms three times as long as those of the lower house. Since we had no hereditary nobility (and were prohibited one by the Constitution) each state's two senators were elected by the state government itself. Thus insulated from the constant pressure of needing to worry about re-election and the public whim of the moment, the Senate would serve as a brake against the spasms of popular fads, and prevent asshattery like legislation proclaiming the theme song from Friends as the national anthem or Britney Spears being voted Dictator-for-Life.
The second, even more important, function served by senators, was as representatives of their state or commonwealth government to the federal government in Washington. Whereas a representative from Dubuque or Des Moines would be voting the whims of their respective constituencies, the senator from Iowa was expected to represent the sovereign interests of the Hawkeye State. This has very specific effects on the separation of powers in the U.S. Constitution.
One of the most telling effects comes from the fact that the Constitution specifically delegates the power to ratify treaties to the Senate. This is important both in the nature of our federal system of government and in the nature of foreign treaties at the time the Constitution was written. In the late 18th Century there were no treaties designating United Nations World Heritage Sites or allowable levels of CFC emissions. Treaties involved war and peace, mutual defense, and the setting of national boundaries. By giving the Senate the power to ratify treaties, this was implicitly acknowledging the sovereignty of the individual states. A mutual defense pact with Absurdistan could not be entered into without at least a majority of the states feeling that they had sufficient ties to Absurdistan to make it worth defending. Likewise for treaties setting borders or ending wars; approval by the Senate was the de facto shorthand means of gaining the approval of every statehouse in the federation.
Once senators became directly elected, it effectively demolished the reasons for a bicameral legislature and the division of powers enumerated in the Constitution. The senator was now removed from his or her lofty perch and made as much a weathervane of the public whim as the representative. Further, the senator from certain states, those dominated by a single large city, no longer represented the interests of the state, but rather those of the small portion of the state in which the majority of the populace was concentrated. The Senate, originally a legislative buffer against the popular whim of the moment and the inexorable demographics of urban centers that dominate the House of Representatives, now became the very same thing it was meant to counterbalance, but with a six-year term instead of a two-year one.
In the years since 1913, the effect has become all too obvious. Treaties are ratified based on pressure from the media, not their agreeability to the sovereign states that are bound by them. There is no longer a legislative brake on popular fads or the whim of the moment. We've bounced from New Deal to New Frontier to Great Society to everything short of the Great Leap Forward. Commentators can make mouth noises about "We're not a democracy, we're a Federal Republic" all they want, wingnuts and moonbats can natter about wimmen voters and the Electoral College 'til the cows come home, but this nation became a democracy, for good or ill, with the passage of the Seventeenth Amendment. And we all know what the apocryphal quote says about democracies:
A democracy cannot exist as a permanent form of government. It can only exist until the voters discover that they can vote themselves largesse from the public treasury.