Now see, I'm conflicted. I do love literature, so I agree with #2 and #3, but to answer objectively, I would have to agree with #4 except that it's about the "what" and the "how." All of us can relate to the idea of family betrayal--either because we could never imagine it in our family or because we could all too well. That is what draws those who aren't into literature to Shakespeare. It is the how then that keeps them and keeps others coming back. The connection to pop culture was a great example. We all knew that Harry Potter would survive each and every book (at least until she wrote the last one)because it is that fact and the curiosity in how she would do it that kept many readers coming back.
Unlike the two previous posters, I am not a literature lover. So I'm not as enthusiastic as they are. What I would say is that Shakespeare's plays are not about the plot -- they're not about WHAT happens, they're about HOW it happens.
But if you think about it, that's not all that much different from modern cultural stuff. Did anyone go to the last Harry Potter movie NOT knowing how it came out? What about Twilight? So it's the same thing, right?
To echo Post #2's response, Shakespeare should never get old to anyone who appreciates a writer's masterful skill with language. Each time I reread or re-watch a Shakespearean play, I find something new in it, whether it be a discovery of another double entendre or a director's creative but credible interpretation of the play.
Additionally, almost all of Shakespeare's tragedies announce their endings near the plays' beginnings. Romeo and Juliet's prologue informs the audience that the lovers' are not fated to be together. In Act 1 of Othello, Iago tells the audience that his plan is to destroy Othello and Cassio and to use Desdemona to do so. These examples of foreshadowing or predictability simply go along with the basis of a tragedy.
I have taught this play several times and seen it performed on stage several times. It NEVER gets old for me, even though I know the story intimately well and can quote the dialogue along with the actors. The reason why is because Shakespeare is the most talented, gifted playwright in the English language. Not only did he craft gorgeous language, but he wove intricate plotlines and created memorable characters. The themes of Hamlet are timeless and never get stale.
Having said all that, the fact that the audience knows the plot in advance simply creates suspense and dramatic irony. We know what will happen; however, we are compelled to read/watch in order to see it unfold.