The questions Comey wouldn't answer in public hearing
When asked if he had thoughts on whether President Trump colluded with Russians, James Comey said, "that's a question I don't think I should answer in an open setting."
Everything else he said he couldn't answer in a public setting:
Why Attorney General Sessions had to recuse himself from the ongoing investigation. (Comey did say the FBI was "aware of facts — that I can't discuss in an open setting — that would make his continued engagement in a Russia investigation problematic.")
Whether any of the contents of the Steele dossier on alleged Trump behavior in Russia have been proven accurate.
Whether former National Security Advisor Michael Flynn is a "central figure" of the Trump-Russia investigation.