The big thing for me is the resource usage: Rails is wonderful, but I have a lot of different apps running on my server, and they all consume a fair amount of memory, which adds up. Anything that will slim Rails' footprint down some will mean a lot of savings for a lot of people.
You can try Passenger and Enterprise Rails combination before multi-threaded Rails.
It's interesting that in his announcement dhh actually downplayed the importance of this contribution, saying it is just another item in the enterprise-readiness checklist.
I'm pretty sure I read somewhere that a lot of Ruby-related native code, including db drivers, is written in a way that actually blocks the green threads when the native code is waiting on I/O. Can anyone confirm or deny this?
