What & Why
In order to understand this, we need to first address a different question, which is why, oh why, do we need yet another runtime environment?
Deno was created by Ryan Dahl. Ryan is the same person who created Node. Ryan points out that when he initially created Node, he was primarily concerned with I/O operations. To be even more specific, he bemoaned Apache HTTP Server, which was the most popular web server at the time. Specifically, the fact that Apache struggled with large numbers of concurrent connections. Also the fact that the prominent paradigm for web server code at the time was code that was executed from top to bottom in order. If any operation along the way took a long time, the rest of the code would be blocked from executing and this affected performance. All to say that when Ryan created Node, he did it to create a highly performant way of handling http requests.
And he accomplished that goal. In 2012, Ryan left the project because he considered it mostly "done". Fast forward to today, and we know that Node has changed significantly since 2012. It's also given Ryan a lot of time to reflect and there are many things that he regrets now about Node.
In his talk "10 things I regret about Node", Ryan looks back with 20/20 hindsight on some of the things in Node that he wishes were don't differently.