The big difference is it's built from the start with web tech.

Looking at it, my biggest criticism is that I would appreciate it more if it did share/piggyback more with graphviz's DOT language. [1]

If it could import/export DOT files it would make it easier to do things like start with Mermaid and migrate to Viz.js or graphviz itself (on a server backend somewhere) if you find for a particular graph you might need different performance characteristics or to use one of graphviz's alternate layout engines (such as a neato or twopi).

(I could especially see a use if it could manage to export its more domain-specific sequence and class diagram formats to "traditional" graphviz digraphs. Nearly all of that formatting is possible with enough work with digraphs in DOT language, but it's not a pleasure to build by hand and a DSL convertable to it would actually be generally useful, I would imagine.)

[1] https://github.com/knsv/mermaid/issues/5 [2]

They have a broadly similar syntax for directed graphs, but Mermaid also provides a simple syntax for drawing UML sequence or class diagrams. You can draw class diagrams with GraphViz, but it's comparatively kludgy [1].

[1]: http://www.ffnn.nl/pages/articles/media/uml-diagrams-using-g...