Also, big "ugh" to browser-based tooling. I want to browse webpages in my browser, I don't want to do my data science work there. We don't even have a good native client for Jupyter notebooks yet, let alone for this new Jupyter alternative that doesn't support the existing Jupyter kernel protocol.
Not only that, but Pluto also apparently has some obnoxious UX limitations that remind me of other less-than-usable wannabe-Jupyter-notebooks (e.g. Apache Zeppelin, Databricks): https://towardsdatascience.com/could-pluto-be-a-real-jupyter...
In short: nice idea, but I'd rather see continued unification around Jupyter and a proper IDE that can at least emit and interact with Jupyter-compatible data.
On the other hand, the Jupyter notebook JSON format is bad for a variety of reasons (e.g. you need special tools for readable Git diffs) and I really wish we had all settled on R Markdown instead. But R has its own NIH tooling problem and nobody was ever going to adopt it because the R community itself (driven by RStudio) has little interest in sharing or interoperability with other languages.
</cynical-angry-rant>