If not for the work of Heminges and Condell, who published the First Folio in 1623, we would have lost 18 of Shakespeare’s plays, and would be stuck with often very poor quarto copies of 18 others: we would have no Macbeth, Twelfth Night, or Coriolanus. Few playwrights were preserved in similar anthologies; Jonson, who oversaw the publication of his own plays, is another exception.
Wanna get upset? Look at the the Lost Plays database to tell us what we’re missing. It has been compiled by David McGinnis and Roslyn L. Knutson. According to this abstract of a presentation by McGinnis–where I found out about this–we apparently lack something over 550 plays written between c. 1580 and 1642, when the theaters were closed. Is part of the reason for Shakespeare’s uniqueness that we simply don’t know enough about his competition?
And being interested in Wycliffite studies, I sure would like to know what the plays entitled Sir John Oldcastle–parts 1 and 2–in 1600 were like!