Warnings are usually given for hubs that are too short, are too promotional (i.e. too many links), or are in some other violation. Next to the warning it tells you what's wrong with the hub and suggests how to fix it.
By looking at your hub, I can tell you that you have too many Amazon and Ebay items and not enough words. I believe for every item you need approximately 50 words or more. There's an entry in the Learning Center about it. Just adding a ton of items at the end of a hub is generally not accepted by HP.