Separation skills and hands above all for the WR. If the receiver can't separate until deep or late, then there are few opportunities for them to contribute. If they can only separate in the short areas, then we have another slot guy - which we don't need. Downfield blocking, size, and height for contested catches are secondary. For the times you can't "scheme a guy open", you want some natural separation skills.
Replacing Davis is thinking too small. Bills need to draft a guy to be the #1 because by the time he "gets it" in year 2, Diggs won't be a dominant #1. Think ahead, not behind. Then, by all means do draft another receiver in rounds 3-5 that has scary speed or high RAS.