Google Check List
1) Website size
It is reasonable to believe that if an index or index page size is too large, many pusers will click away so may loose relevancy to show in the top results.
2) Outbound links
How many do you have on your index page?
3) Inbound links
How many does your Website have? If it is vastely more than internal links then it could be seen as a 'parked page.
Since it may connect innocent Websites with scraper Websites.. do you use it?
5) Content updated regular?
Some Websites do not have content updated too much because they offer tools over info, but google may consider Websites with rare content updates to be poor quality and drop positions for them.