Based on the description and rules, it seems like just about a review of anything is fair game. I think reviewing a book is one that can be done with no questions asked.
I think HubPages wants us to think about reviewing both mainstream items (like a product, i.e. appliance) and things that might be more out-of-the-box thinking (like a comic strip or something).
This contest seems really cool. It will make us think about the things we do and use every day. I just hope I will be able to get a lot of articles done. Time is not on my side!