Did you check the help section when you first got you account? It explains how to get one from reviewing a game and one of the admins made a thread about merits and GentlemanClam's link shows how to get a merit.
How a proper merit review looks like is 100% objective, but the link that GentlementlemanClam provided explains the process thoroughly. And when your review actually gets merited, it is shown in bold for all, especially the developer, to see clearly. So you can consider one of the great things about having merited reviews, besides a 25AP present, is that yours is selected for a personal recommendation for the developer to read.