In this article from Bitcoinist you can find more information: http://bitcoinist.com/12-cryptocurrencies-avoid-weiss-ratings/

Weiss Ratings scale is from A to E and is taking into account: investor risk and reward plus tech and adoption fundamentals of each coin. A is the best rank position they give, D is considered “weak” and E is “very weak”. Also they are updating its rankings frequently.

They already give reports for Bitcoin with a C+ and Ethereum with a B, and they include the specific reasons for this punctuations.