Honest question: What are the main selling points of Io ? If I wanted to compare its strengths and weaknesses against Go, Rust or even a less popular language like Nim, where could I find that out ?
Its incremental GC makes it somewhat suitable for game scripting. It uses a single OS thread with coroutines, like Lua, although it supports multiple VMs in the same process, so asynchronous message passing between two VMs on separate threads might be easy enough to hack together.
It's good for people who like the minimalism and dynamic nature of the language and what it lets them accomplish, so long as their task isn't CPU-bound and doesn't require a rich library ecosystem.
Generally if you're asking this question then you should just stick with the more well-known platforms.
Languages themselves don't have strengths and weaknesses. You need to compare everything else along with the language, the community, library support, stability of the implementation. And if you're looking for a comparison of these in a brand-new language, what you're looking for is not something it can offer. There is no community, libraries will be extremely spotty and not well documented. You also need to know the language it's implemented in, too, because you might need to debug the odd bug in the implementation yourself.
— the language is very mutable, almost all behaviours can be modified
— it's simple, all you can do is send messages to objects
— you can write DSLs and macro-like things without special macro support
— it's so dynamic you'll end up live-coding without realizing it
It has less syntax than Smalltalk, it's homoiconic like Lisp, but doesn't need support for macros because of the way messages are handled.
A not intolerable way of thinking of Io is it is to object calculus what Lisp is to lambda calculus. There are parts where that analogy breaks down but it's not a bad first approximation.
edit: formatting
Apparently it's supposed to be really fast, too, for a pure-interpreted language, but unfortunately the name 'io' is unsearchable and I haven't found any references. (I think this is a major factor why nobody's ever heard of it.)
As with Go, appending 'lang' gives more luck: https://google.com/search?&q=iolang. (I still didn't find a reference for the speed, though.)
Io is no speed deamon, but for heavy computation, its easy to dip into compiled C or C++ libs, putting it in the same ballpark as an unjitted ruby, python, or lua.
Looking back, I wish I had written Io in Lua.
No, seriously, thank you for creating it. Although I hadn't had the chance to use it in production (most of my stuff is just numerical), Io is an incredibly enlightening language.
So learn it to be a better programmer.