TLDR; syntax, the community, rails, quick feedback & prototyping, mostly what I'm used to... I also think other languages and frameworks have caught up on syntax/features so the gap has closed
I'm coming from 20 years and have travelled through PHP, C, Java, a couple of .NET languages and then have been on Ruby primarily the last 7-8 years. I've continued to do PHP/C on the side.
I think it is important to say that most Rubyists come from the Rails framework which until the last 1-2 years was without question the best framework for developing any sort of prototype web application and to some extents anything that required database access. I think this is probably important to keep in mind as I think the love of Ruby is mostly tied to how well Rails stacks up to other frameworks.
I did have the opportunity to jump back to C# about 6 years ago and decided to stick with the Ruby train.
Primarily there were 2 reasons I did so:
The first point is obviously really subjective. To be honest given I work in JS, PHP & Ruby regularly I know if I was forced to write Java or any language for a month I'd likely feel differently.
What I will say is that as a part-time C-ish developer and watching some of the Java and .NET developments it seems that Ruby has been quite stagnant from a syntactic POV whilst changing dramatically under the hood. Syntax is our UX and so if a language excels at syntax and structures then it will always seem 'nice'. Other languages have moved on syntactically and structurally (some driven explicitly by the popularity of Ruby/Rails like .NET and PHP) so it has become less of an issue.
I think if you give Ruby the time to get over that initial period where humans dislike change then Ruby can give you quicker feedback than pretty much anything else out there. Certainly quicker than most if not all C/Java type languages.
As to the second point this is just something I've found as you get into the Ruby community. It can quite easily be because I haven't been embedded in the Java community a while but I find Rubyists to care more about code than other communities. It is hard to explain but I found way too many grizzled old C/Java/PHP/etc devs just getting through the day compared to Rubyists.
As of the last 5 years there has also been a big revival in the Ruby/Rails community about REAL OOP which has been fantastic. It wasn't uncommon to see Ruby gain optimisations that were a decade old in other languages or presentations by developers on 'new patterns' that were also a decade old if you were paying attention. That seems to happen much less now.
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