A lot of people (on HN at least) seem to think it's a problem that the First Amendment only applies to Congress, letting companies like Google or Facebook censor speech as much as they like.
The Second Amendment isn't even written in complete sentences, and we need a room full of scholars to figure out just what exactly they meant by "A well regulated Militia,"
The Third Amendment just seems absurd in the modern day.
Maybe we don't need the Electoral College?
Maybe the President's powers to draft executive orders and pardon people undermines the legislative and judicial branches and should be done away with, given how often they've been abused?
It must be possible to have a more efficient census than through the process required by the Constitution.
It should be possible, in a nation of immigrants, for an immigrant to run for President.
There is no Constitutionally guaranteed right to vote. Amendments related to voting, yes, but nowhere is voting considered to be a right, as opposed to a privilege.
Do we really need to explicitly make the justice system an exception to the Thirteenth Amendment and banning slavery?
That's all I could think of off the top of my head, not being a constitutional scholar. But it stands to reason that a document as old as the Constitution should be updated more often than it has been.