One-person-one-vote is a terrible system, with the only advantage of it being simple and easy to understand, which was important when votes could be counted only by hand.
CGP Grey has made an excellent and well known video series explaining this, so I don't think I need to debate it much further here.
CGP Grey Voting Series: http://www.youtube.com/playlist?list=PLej2SlXPEd37YwwEY7mm0W...