Since the Senate is very large and Senators are directly elected, it is unclear why there is a Senate — that is, why the federal legislature is bicameral. Bicameralism increases the transaction costs of enacting legislation, which can be good or bad (it is bad in national emergencies, as in the financial crisis of last September), and it also increases the cost of repeal, which on balance probably is bad, arbitrarily enhances the political power of sparsely populated states, results in many unprincipled and confusing legislative compromises, and diffuses responsibility for legislation. It is not clear that on balance we are better off with the bicameral system.
The filibuster is an incomprehensible device of government. A supermajority rule, whether it is the rule of unanimity in criminal jury trials or the supermajority rules for amending the Constitution, makes sense when the cost of a false positive (convicting an innocent person, or making an unsound amendment to the Constitution) substantially exceeds the cost of a false negative. But it is hard to see the applicability of that principle to Senate voting, given the other barriers to enacting legislation.
One’s anti-Senate sentiments should only be bolstered by the fact that the historical origins of the Senate are perfectly clear. In particular, the arbitrary enhancement of the political power of low-population states was undertaken as part of a non-mysterious raw political compromise. But that was hundreds of years ago, and in the intervening time the gap in relative populations between states has grown much larger.
On the filibuster, Posner gets at the crucial point. Even without a supermajority rule in the Senate, the United States would still feature many more veto points at which legislation can be blocked (you need concurrent majorities in two legislative houses, plus at least two committees, plus the assent of the president) than most advanced democracies. There’s no systematic reason to think that this feature of our system is conducive to the public interest over the long term.