Here are some of my first impressions, in case this might be helpful to the devs.
UI is clean and minimal, which is amazing considering how many features are available
In-app documentation is very useful and thorough. I suggest making this available online, so people can learn more about the app without needing to install it first (if it is already available online, I suggest improving discoverability). I did struggle with a few bits, possibly due to English not being the dev's first language, however nothing major.
Navigation/structure was a bit confusing at first, but eventually made sense. After clicking on the "Please start here" doc and reading the content there, I wasn't sure what to do next. Clicking the links led to more content, however I was confused about where this content was actually located (this could be made clearer by making the Doc Tree expand to, and highlight, the doc currently open - Edit: I just learned this can be enabled in Settings; I suggest having this be default). At first I didn't notice that these links opened in new tabs, however that wouldn't have helped in terms of confusion. I had noticed the ">" next to the doc name in the sidebar, however I wasn't sure what I'd find there. Once I did, and clicked on the sub docs, I finally understood the logic - that the Notebook is made of docs, that each doc can have content, and that docs can be nested.
- Elaborating a bit on that, a lot of my confusion is due to the fact that I expected the hierarchy shown in the sidebar tree (the "Doc Tree") to be reflected within the documents themselves. However this is not the case. For example, going in the doc "Please start here" (under the "SiYuan User Guide"), I'd expect to see its sub-docs in there somehow (it was extra confusing that the first topic within that doc was actually "🍫 Content block"). This is especially confusing with docs that are empty (ie. being used only as a folder), such as the "🍫 Content block" doc. Although I could eventually get used to this behaviour, I'd rather have the option to show a list of sub docs at the bottom of any doc that has them, just to make sure that content isn't missed.