From what I know, these are the criteria:
1. The hub is uniquely very interesting and useful. So choose a topic that will engage a huge number of readers without being too commercial or over-promoting.
2. The hub should be well-written, no misspellings, grammatically correct.
3. The lay-out is awesome. All capsules -- pictures, ads, text, comments, etc -- are in symmetry and complimentary.
4. The hub of the day is chosen by the HubPages staff. So, not only should your hub be put forward to impress, it should also be your total performance as a hubber. You should keep in mind that HubPages is a community.
Good luck and God bless!