From the "How can a post be deleted?" faq answer:
The system will automatically delete unlocked, unanswered questions with score of zero (or one if the owner is deleted), fewer than 1.5 views per day on average, and fewer than two comments after 365 days.
What should we do with questions that won't be deleted as they have more than two comments? 'Cause they'll just flow to the end of the unanswered tabs.