I refuse to name just one thing. I can't rank a number of ideas by how important they were relative to each other, they were each important in their own right. So, to preserve the voting format, I'll just split my suggestions into several comments.

Some notes in general. The first year I used to partially misinterpret some of your essays, but after I got a better grasp of underlying ideas, I saw many of the essays as not contributing any new knowledge. This is not to say that the essays were unimportant: they act as exercises, exploring the relevant ideas in excruciating detail, which makes them ideal for forming solid intuitive understanding of these ideas, a level of ownership for habits of thought without which it hardly makes sense to bother learning them. Focusing attention on each of the explored facets of rationality allows to think about extending and adapting them to your own background. At the same time, I think the verbosity in your writing should be significantly reduced.