Fastidious attention to detail, it all starts with the maps or shaders. Spend time looking at work done by professionals at ZBrush or CGsociety to name a couple. Properly scaled textures are a must, I see people putting up renders all the time with clothing or wood textures that are not properly sized for the object they are placed on. Or wood grain textures running the wrong way. Its usually not just one element that makes it pop realistic

but a texture/shader that works with the lighting. You can get very good renders with basic light setups if you use material setups that work with them. Experiment, start with just 1 light with all the rest turned off. Try no lights with just an emmitter (a prop ball scaled to about 800%, white, with ambient set to about 5 can give good results, then duplicate and try several prop balls placed in diff positions) Just my opinion but I see a lot of vendors putting up product with what appears to be minimal work on textures, relying on some of the new free shader and lighting setups to make up for a lack of work on textures. Imagine if "artist" would use these products to inhance their work rather than use them as a shortcut. Getting something to look professional just flat out takes a lot of work. Read and study books or articles on photography, lighting, etc.