A lot of people have suggested that the thirteenth and nineteenth amendments (abolition of slavery and women's sufferage, respectively) should have been included. After some thought, I think their actually, at least in one sense, less important than, say, the first and fifth.
The thing is, every modern liberal democracy (that's the political science use of liberal, by the way) has an equivalent to the thirteenth and nineteenth amendments. Every western European nation has ended slavery and given women the right to vote, along with Australia, Canada, along with most (all?) of Central and South America and even most of the democracies in Africa and Asia.
The first and fifth amendments, among others, however, are essentially unique. The European nations don't have a first amendment, and in fact do a number of things which in the US would be considered to infringe on free speech. To give one example, England's libel laws are brutally tough, enough so that they are a definite limitation on open discussion. There are plenty of others.
The fifth amendment (my personal favorite) is similar. That included the vastly underused prohibition on eminent domain; i.e. the government can't take your property without paying for it. It also has due process rights and others.
Most other countries don't have the bill of rights, or anything resembling it, and in many ways they probably suffer for it (at least in my opinion). It's probably the Bill of Rights, more than anything else, which makes the United States unique.
So, while the abolition of slavery and women's sufferage are certainly great and important amendments, it's probably safe to say that they aren't special. If they hadn't been passed when they were, they would have been passed some time later. That's not true with most of the Bill of Rights.