IMHO - nothing.
I've been a professional programmer for over 20 years, and it's no harder than any other language I've ever seen, and easier than most.
I actually started Ares in Python and had a hard time getting it off the ground. Python is way more finnicky about little things like indentation that can throw off folks who aren't used to programming. It also didn't support the dynamic loading properties that I needed for Ares. It's a fine language overall - I've got nothing against it. It just wasn't the right tool for the job.
The only genuine criticism I've seen about Ruby is that it has many ways of doing the same thing, whereas other languages (including Python) are more opinionated and force you to do things a certain way. I mean... I guess I can see where that might throw some people off? But personally I find that as a selling point, not a down side.