What was the most important new factor in the success of Langstroth's hive design?

The precise spacing determined by Langstroth was 1/4 - 3/8 inch (6.4 - 9.5 mm) between the frames and between each frame and the side walls of the hive.

The space is popularly called "bee space". It allows the frames to be easily lifted out without too much disturbance of the bees. And it allows the bees to move and work around and between the combs as they construct the wax cells and fill them with honey.

The Langstroth bee space of 1/4 - 3/8 inch (6.4 - 9.5 mm) has remained the standard in most countries, although some variations exist e.g. 3/16 - 5/16 (4.8 - 7.9 mm) in Britain.

Langstroth knew that when he left a small a space (less than 1/4 inch or 6.4 mm) between the combs, or between the combs and the sides of his hives, the bees would fill it with propolis ("bee glue") thus cementing the combs into the hive. On the other hand, when he left a larger space (more than 3/8 inch or 9.5 mm) the bees would fill it with comb which had a similar effect.