Re: wedding photos: mediocre vs
Having worked with and learned from the best in the wedding business I would venture to say that caputuring people in flattering natural looking poses in flattering light and isolating what is most important from the background clutter are the main qualities which separate the "wow" from the rest.
Single flash, no matter how it is sliced, diced or bounced is flat booring mediocre light. Using more than one flash, if only to park it behind the action for some rim lighting, adds the illusion of depth and will help a flat lit situation come alive. I was fortunate to learn this 35 years ago and have used two flashes every since whenever possible. When one is on a camera bracket and a second is on portable wheeled stand (I use a modified medical IV stand) shooting with two flashes is as easy as using one and makes candid shots like these possible, even at crowded parties and receptions:


Those same basic tools were also used for the shots in these slideshows:
http://bossa.nova.org/slideshow101/
http://bossa.nova.org/slideshow102/
But light alone is not the solution. One needs to know how to use it effectively to flatter the people being photographed. That first requires the abilty to quickly recognize which facial angles are the most flattering, then knowing how to position the light to draw attention to the front of the face, not to the ear on the side of the head or some bright distraction.
For example, in the second shot above it is not just a happy accident that the subject in the photo - a transferring pastor from our church at his goodbye party - is captured in a flattering short-lit pose. I wanted to capture him framed by the people he was talking to. The direction he was facing when talking to the guy with the back to the camera was predictable, which allowed me to roll the off camera light over to the left so it was 45 degrees off his nose when looking at him. I dialed back the ETTL ratio to avoid blowing out the foreground. It took just seconds to set up. That type of shot would be impossible with a single flash.
The above examples utilize flash, but the same thing applies to natural lighting or ambient artificial light. First you need to identify the dominant ambient light source, determine its direction, decide how best to use it (key light, back light, fill for flash, etc.) then find camera positions where the action can be captured using that lighting strategy effectively.
In a nutshell, you simply need to make people in the photos look good and like they are having fun there. That requires mastering the craft part of photography to the point it is second nature, which frees one to focus on the more important part; making the people look both confortable and good in the photos...
CG