I like RoR but "Ruby on Rails" and "modern" in the same sentence seems kind of funny.
But then again, "modern" is subjective in itself and most of the websites I see these days (even built and maintained by large companies) seem pretty ancient.
No semantic HTML, still using divs everywhere, no accessibility, all these useless third-party dependencies and lockins vs the new APIs being introduced natively in the browser every day, ajax, jquery instead of using the web platform, hell-- most web developers don't even know what a dialog element is.