Relevant subject matter, proper grammar, accurate spelling, easy to follow instructions, interesting and original/informative pictures, and a personalized or unique take on a subject/story. From what I have noticed, most "Hubs of the Day" have been interesting ideas to try (cooking, games, etc.), or they have been how-to or editorial-like reports. I would never write an article to specifically get the hub of the day. It stiffens your writing too much. I would say look at the style, format, and voice of those articles you like to see what you think makes these hubs unique and original. Write out a list, and keep that list handy when writing your next hub. It may gives you a few ideas or some helpful tips as to how to narrow your topic, writing styles, etc. Another idea is to follow the top hubbers to see what kinds of things they are doing...and follow their excellent example. This is what I do when I am having difficulties or need some inspiration.