What's nice about Ruby is that there is a huge ecosystem. Ruby on Rails is already very complete (especially compared to PHP). There are a lot of plugins for Rails that add the functionality you want. For example: adding image uploads (including thumbnail generation) was about 15 minutes of work, adding comments to my models too, and voting on models/comments was also done by installing a plugin. In PHP, there's a lot of code available, but most of it doesn't integrate automatically with other code.
I think that the amount of smart people that are working on a language/framework is very important. In Haskell, for example, there are not too many people working on web frameworks, and there's still a lot missing or broken. In PHP, there are a lot of people working on the language/frameworks, but a lot of the smart developers have moved to other tools, such as Rails/Django/etc. Therefore, I would just follow them and use a framework like Rails.
(A slight warning: this is all very subjective, this post is not based on any objective facts)