Conceptual reality, mettā, ignorance According to some commentaries/abhidhamma, all unwholesome states are rooted in ignorance; that ignorance is, according to Pa Auk Sayadaw, the ignorance that sees things as concepts, e.g., a man a woman etc. However, it seems like in order to do mettā and similar practices, one must take a concept as object. My question: Since wholesome states like mettā are said to be incompatible with unwholesome states like ignorance, why does it seem like mettā is rooted in ignorance?
The important thing about metta is not the "object", but the result of the practice; it's point is so one becomes kind when kindness is not present. 

Notice that Saying benevolence (metta) is rooted in ignorance if a particular practice is described in conventional terms is like saying dhamma is rooted in ignorance because it's also exposed in conventional terms.

Also, as a side note, Metta practice with a person as an object is a later development. 

It would be problematic if, in this or any exercise, one nurtures and develops a wrong view (e.g. believing that a person has an eternal soul).

I'd guess the Ven. meant "seeing things as concepts" as something very specific: substituting reality for some concept. Conceptual thinking is largely intrinsic to thinking. it's problematic because it requires skill to not be confused, to not take something imagined as something real, to not develop wrong view (e.g believing an object is detached, eternal, subsisting without conditions).