Q149. "What are some of the weaknesses of the Constitution?"
A. Of all the questions asked about the Constitution, this is actually one of the most common — the most frequently asked. The problem with finding an answer is that it is no simple question. If you asked an Anti-Federalist what were some of the weaknesses of the Articles of Confederation, they may not have been able to come up with a single one, but we can come up with many, given our historical perspective.
That having been said, let's discuss the weaknesses of the original Constitution, unamended. First, it had no bill of rights. Bills of rights were common in state constitutions at the time, but the Framers felt one to be unnecessary and even dangerous to liberty. However, of all the arguments of the Anti-Federalists, the lack of a bill of rights was one of the most compelling, and the omission was quickly corrected. Second, the method of presidential selection, with the first place candidate becoming president and the second place candidate becoming Vice President, was flawed, mostly because the Framers failed to foresee political parties. The flaws were spotted quickly, and fixed with the 12th Amendment.
I consider the protection of slavery to be a major flaw. The Constitution probably could not have been a success without some protections for this infamous practice, but it didn't really do anything to make things any better. At the least, it could have mandated a phase-out of slavery over several decades.
Some consider the vagueness of some constitutional provisions to be a flaw — others consider that a feature. Regardless, there are places where the Framers could have been more clear, but usually those are evident only in hindsight. You can see what some others have felt are weaknesses by looking at how some have wanted to replace or amend the Constitution.