The most notable for all Vim users is probably forcing upstream vim to merge async support for plugins after refusing it initially; albeit with a different API. (there is quite some bitterness on both sides of this issue, I think)
The other big win is providing a embedding API so other editors can be built on top of the core editing engine. Which resulted in multiple interesting GUI projects which you can find in other comments.
Others are Lua plugin support, good terminal integration, way better default settings, ...
That's more to do with how you setup to trigger the commands.
Async here really means that it doesn't block with your typing, jumping around in the text editor if plugin starts doing some heavy work in the background due to some action that got triggered by whatever event you had setup for plugin to do. One of the plugin that I particularly remember was syntastic (I haven't kept up with it to know if they've fixed this or not) that would just create janks every now and then if the workload was heavy.
The other big win is providing a embedding API so other editors can be built on top of the core editing engine. Which resulted in multiple interesting GUI projects which you can find in other comments.
Others are Lua plugin support, good terminal integration, way better default settings, ...