I would say that you are best off making things that other posters either haven't thought of making or would see as a considerable investment of time.
I don't really register pixel art unless it is actually original art rather than just copying a sprite block-for-block from somewhere else.
Redstone engineering is impressive, but it is a bit of a niche that I don't look at unless it is something that I might need to do something else.
Buildings are good, but you aren't going to get people to download a starter house they could build themselves in an hour. It needs to be immediately obvious what it is from the first image, and that image itself needs to be impressive to warrant a second look.
Size alone is not impressive. It is easy to make big shapes in McEdit. People appreciate complexity and attention to detail. The Medieval Buildings pack is the best example I can think of. It gathered a massive amount of attention because of the craftsmanship involved. None of them were very big, but they were presented well and did things with blocks that many builders had never thought of.
I think people tend to key in very strongly off of the initial image, as I said. This represents a bit of an issue with projects that you post long before completion. If I see someone post about building the Death Star in Minecraft and they haven't started, I am not going to take a second look. But if they have a picture of part of the project and it looks good, I will probably follow the progress.
Obviously, having anything listed as a Featured Project on the front page dramatically increases the attention that it gets. Otherwise you only have a narrow window between when you post it and when it rolls off the front page, or unless someone is browsing or looking for it specifically. If a project is popular, it gets featured. Being featured makes it far, far more popular.
