Good god, people. It seems everyone has an entirely different idea about what they mean when they're referring to Luffy's mental efficiencies.
First off, to those who believe him to be mentally retarded: I can say from experience and years of working within the Sp. Ed. community that he by no means has an actual cognitive deficiency. He learns quickly and understands most concepts that are explained to him without a condensed explanation. He's simply uneducated, as evidenced by the fact that he doesn't read the newspaper or that he was raised in the wilds by his grandfather Garp.
As for those who believe he has Attention Deficit: I'm going to keep this one simple and point out that as of late, everyone is being diagnosed with ADD or ADHD these days. Overdiagonosis means you'll see the symptoms everywhere. He's just flaky, that's all.
And to those who refer to him as a genius and therefore rule out retardation: know that in all families who have a member with genius IQ levels has a DIRECT blood relative with autism, Asperger's Syndrome, or some other mental retardation. In fact, many people with IQ genius tend to be afflicted with Asperger's. The link between the two is accepted to be genetic, and some believe that they ARE the same gene. So don't rule out the fact that even though he shows genius intellect, he may still have to ride the short bus.
My apologies if that post was too long/complicated. I just HATE it when people aren't properly aware of mental affects.