I was attracted to Obsidian originally because it worked with files. Plaintext files which are easily read by, it feels, nearly all programs. That attraction was undermined when I understood that Obsidian meant only .md files as most of the programs that I used preferred .txt. More recently, my preference for files has expanded to word processor files - .docx, .odt, .rtf; despite the claims from markdown activists about plaintext being the only eternal format, I have found word processor formats to be robust and widely interpretable in use - even files that originated decades ago. And these formats have the advantage over markdown in that they are *complete* - they contain their own images, have colour and instructions for printing; markdown needs multiple files, CSS and HTML to achieve the same capability.
Be that as it may, the first, greatest idea I learned with Obsidian was Nested Vaults. I happened on the idea very early, realised the benefits, tested out the risks (because the developer warnings were so strong and stridently repeated), and have used them ever since. In practice the risks are minor and small - far lower than the sync risks that most Obsidian users run regularly - and easily managed by those with a modicum of sense. I soon learned to add all files to a vault, not only those that Obsidian could read; this is great for file and project management because everything required is always contained within a specific folder.
The major advantage of nested vaults is essentially focus. There is some similar advantage to nested folders, but it is slight so I'm not surprised that I hadn't really taken to it. But within a program like Obsidian, the advantage is supercharged because the vault system restricts every action to files that are available in the vault. That includes linking, search, tags &etc &etc. Which then means that tags, star files, links can be constructed with just that vault in mind.
I tend to set up a vault for each project. As I develop and then home in on a project, there might be progressively more restricted vaults. Because it is a single system of nested vaults, it is easy to switch focus to a different area or to a much wider area of interest. I suppose it is, in effect, an indexed filter system.
As I shift towards predominantly using Tangent Notes, I am very pleased to see that its architectural similarities to Obsidian include the possibility of nested vaults (nested workspaces in Tangent language). In some ways, it takes the idea further by having a card view of the files within a folder - so I am structuring my vaults/workspaces now to take advantage of this functionality. There are many features still to be added to Tangent Notes, but it is easy to switch to Obsidian or other programs for those.
At some point I will write a review of Tangent Notes. Though I'm not sure where I will post it (my reviews here are intended for donationcoder rather than a wider audience), since I might want it to publicise it a little if my views on TN turn out to be as positive as I believe they will. I will at least post a summary review here. But this will be at least a month or two away. Might only be a first look, but I'm pretty slow even with those.