@KeithC; Yes, that's exactly it, you actually put it better than I did!
In the shareware version, there will be 6 complete levels, which add up to a playable game, which reveals a significant part of the overall plot/story of the full game.
The only other thing that is likely to prevent my game's entry as a BOTB, is the lack of in-game video. This was a deliberate decision on my part. I wanted the story to be told in real-time, without the interruption of cut-scenes or video, in the style of Half-Life.
I've used the gameplay and level design to tell the story, with NPCs giving vital clues as to what to do, or where to go next. I also tried to use the levels themselves to tell the story. Each level has a visual theme, which provides further clues as to what is happening story-wise.
Because of this decision about in-game video (or lack of), I'm aware that my level design has to be done very carefully, with a lot of thought and attention to detail, to get my game anywhere near the kind of standards that are reqired for BOTB status. Therefore, I'm still a long way off even considering entering it!!
On the plus side, I believe my game conforms to all the other requirements, so we'll just have to see won't we? lol