During Birkat Kohanim, each and every Jew must stand opposite the Kohanim and have kavanah to accept the blessing. He should not look at the Kohanim or at anything else, so as not to distract his thoughts from the berachah (Shulchan Aruch 128:23; Mishnah Berurah 89).
The Chachamim established that a Kohen who has an abnormality on his body that would likely cause people to stare at him, and divert their thoughts from the berachah, may not ascend to bless the people. Therefore, before the Kohanim became accustomed to covering their faces and hands with their tallitot, any Kohen who had a deformity on his face or hands, such as unusual facial freckles or crooked fingers, would not ascend the duchan to bless the nation. However, today, when all Kohanim are accustomed to covering their faces with a tallit, a defect on one’s face or hands does not prevent a Kohen from reciting the blessing. Nevertheless, even today, if the Kohen has a noticeable deformity on his leg, he should not ascend the duchan to bless the people, for it might distract the minds of the listeners. However, if he is a permanent resident of the community, such that his defect no longer makes people curious, he may ascend to bless the nation, for it most likely will not distract the listeners (Shulchan Aruch 128:30 and 31).
Similarly, a Kohen who has a strange accent, such as one who confuses the pronunciation of the letters alef and ayin, may not bless the nation, since such a thing will distract the thoughts of the listeners from the berachah. However, a person who speaks in the accepted accent, even if he does not emphasize the ayin and the chet properly, does not distract the listeners’ thoughts. Likewise, all the known accents – Ashkenazic, Yemenite, etc. – do not divert attention from the berachah because they are known and familiar. Only someone who greatly confuses or garbles the pronunciation more than is acceptable, or stutters excessively, may not bless the people (Shulchan Aruch 128:33; Mishnah Berurah there).
In summary, the Chachamim enacted that any Kohen possessing something that might distract the people from their kavanah may not ascend the platform to bless; from these laws we understand how important it is that the listeners concentrate during Birkat Kohanim.