RubyMine is pretty damn good, especially because you can immediately jump to the source code, even of libraries, and just read the list of available methods. It's amazing to me how completely Dwarf Fortress -- um, I mean vim and tmux -- has conquered the Ruby dev world.
My analysis: layering. YAML doesn't in itself execute untrusted code, but it has a bunch of semi-experimental features most people don't know about or use, and one of those features (deserializing arbitrary classes) has a side effect that sets values on certain other classes -- not written by the people who wrote the YAML parser -- that then, often later on in their lifecycle, execute this untrusted code. I'm not saying this as an excuse -- I have long mistrusted YAML's blasé complexity, not to mention Rails' anarchic pass-HTTP-params-directly-into-the-DB pattern -- but as an explanation of how they missed it.
Also, more than other communities, Ruby has a cultural gap between the people developing the language and core libraries and the people using it to write web apps and frameworks.
If you’ve ever used the iPad or iPhone, you’ve probably noticed the absence of scrollbars. They don’t exist! Mobile Safari lets you "scroll" the entire page by sliding your finger around, but all other scrollbars are simply discarded. Essentially, we needed to reinvent the native iPad/iPhone scrolling, flicking, and bounceback behavior in JavaScript. Here's how we did it.
Erector is a "builder pattern" view framework, inspired by Why's Markaby. In Erector all views are objects, not template files, which allows the full power of object-oriented programming (inheritance, modular decomposition, encapsulation) in views.