Andy (Crimea River) has described exactly what is looked at, and it's as simple as that - not that the judging is simple!
The decision on which Category to enter is purely down to the entrant, although if we judges feel an individual should consider moving up to the next Category, we may suggest it. But the final decision is still with the entrant, we can not, and will not, automatically move an entrant's category.
A little tip to perhaps improve, or build on skills (pun not intended!) is to look at the four areas, described by Andy, used in judging, and treat these as individual stages of the build. Attention to detail in the construction stage - that is, alignment of parts, seams and joints etc, not extra detail added - sets the measure for the rest of the build.
The application of paint to the best of the individual's ability takes things to the next stage, etc etc.
All of these contribute to the final appearance and visual appeal of the finished model.
It's pointless having the most detailed cockpit, right down to a tiny map in a wallet, if the overall finished model looks like it's been painted with a trowel, over lumps, bumps and visible seams, and the decals are all wrinkled and mis-aligned, for example.
Hope this helps.