There are different criteria for both search engines. Google, the most popular, has a more complex algorithm when its crawlers get into our hubs. As far as I can tell (and I'm no expert!) Google really likes original content, fresh ideas, occasional updates and sensible SEO. There are other factors involved, like word count, duplication, links and so on.
Yahoo or Bing sees things differently. It can tolerate haphazard SEO, deals with Flash much better than Google and has a different approach to links. You can get much more information if you look up comparisons of Bing-Yahoo and Google - I'm only a novice! It's a bit frustrating to find that you have a No1 Google rank with a hub but on Bing it is nowhere to be seen!
Best of luck.