What makes Roam Research different is "block addressing", i.e. you can get a reference to a single paragraph and use it in other docs (either referencing it or embedding it).
(and of course a nice UX for editing)
Side: I've read books and articles in an attempt to understand what sets it apart from other techniques.
I know Niklas Luhmann used it to write tons of books and articles, while others claim to have been able to use it as a "second brain".
But I'm still having trouble understanding how to deploy the Zettelkasten technique effectively -- a lot has been written about the technique, but not much has been written about actual use cases where someone has used it to accomplish something useful, which they couldn't have as easily or at all without Zettelkasten. Due to the lack of examples, I'm trying understand by analogy, by comparing Zettelkästen to something familiar like Wikis.