It's a funny thing with the photos - I always used to use scenic settings for competition entries, but then I noticed that it's the 'clinical' blue or white backdrop shots which always seem to win...
In general, true. I think that when you use a scenic setting it must not detract from the miniatures themselves. That is, they must complement the miniatures but not compete with them.
Trying to "quantify" winning at Steve Dean's I think is a pretty difficult task...
One of the things I have noticed is that folks seem to gravitate towards those entries with a high "wow" factor. For example, a beautifully free hand painted, intricate banner. Intricate patterns on uniforms and shields. A giant which actually looks like it is real. Or as in the last competition, the extraordinary skin textures of the Numidian woman. Baring some unusual entry with a high wow factor folks seem to gravitate towards high wow factor subject matter. Napoleonics, for example, fails in this category for its pomp and splendor, however, they win, only if they can bring that grandeur to reality. Single mounted figures normally do well here.
You have a difficult audience there so it is really hard to say. But I think if you look at most of the past winners you will find some aspect of the painting which makes folks say... Wow.