"enhancements" feel like monkeypatching in that they let you attach things to existing classes for pretty code. This left me sweating at first -- implementations of features like this in ruby seems to be a large part of why the language is both challenging as you add devs (I recently saw one class spanning 4 files and 3 gems, I don't even) and frankly slow, and scala has similar performance barriers brought on by 'implicits' [1] and their ilk. But in gosu, enhancements appear to be defined in an statically disbatchable way so there's zero runtime timewasting, and they're also only valid for your syntactical convenience, they don't complicated inheritance or interface fitting. Looks like all the beauty, none of the sticky mess. Very interesting.
Compatibility looks like a better story than some of the alternatives -- both groovy and scala always felt to me like their mechanics encouraged an extend+extinguish feeling to their libraries because of the radical overhaul or complete dismissing of types, whereas gosu looks like you could sanely call it from java. I (as a purely personal observation; others may disagree) found I could never write a groovy or scala library and ask a java developer to use it with a straight face, and with gosu, it looks like maybe I can!
I'm speaking as someone who's just seeing the language for the first time today, so perhaps my enthusiasm will be modified by practical experience later... but I'm seeing a lot to love both in the examples and in the general philosophy of pragmatism. A language that lets me do more with fewer characters while not giving up ground on compile-time checks and performance is exactly what I want to see more of. I look forward to playing with this!
If any gosu developers are here, my biggest single beef right now is I can't instantly see from your docs what my process is like if I'm going to ship a jar. I assume it can be done and it's not hard to script, but it would be great to see up front, because if that isn't smooth, then my impressions of using it to ship a library usable from other java projects (or feed it into cross compilers, etc) is impacted. I found some docs at [2], but it appears to mostly talk about shell script style usage; ctrl-f for "jar" hit nothing. Perhaps I just haven't looked hard enough yet :) (Also, that help site is one of those annoying things where there's no content without javascript, and even after allowing it, middle click gives me some sad snippet of javascript instead of opening the page I wanted. The rest of your site looks great; I hope the wave of design refresh is going to wash over here soon too.)
[1] https://stackoverflow.com/questions/3606591/why-does-intelli...
[2] https://gosu-lang.github.io/doc/wwhelp/wwhimpl/js/html/wwhel...
Edited: to note that my spate of points-to-love at the top is by no means exhaustive.