Validation in RoR is very simple. ActiveRecord is very nice and a capable O/R-Mapper.
But the testing features are the best. You can easily make unit-tests with fixtures (defined in YAML or CSV files) which will be written into your test database.
You can test your controllers very easily - because it is well integrated. In the coming 1.1 release you get easy integration testing features too.
AJAX support is integrated with ruby code.
MVC is very clear and easy to use - and you get a good design of your web application.
Caching functionality is easy to use.
It´s a collection of best practices for OO agile development. The point is that a normal developer does not care about complex persistence, caching, testing, ... topics. The developer can have the focus on the domain of the application.