Keystone allows the usage of MongoDB since it comes with Mongoose, the most popular ODM for node and Mongo, this means that anything that is built using Mongo can be built with Keystone.

Express comes out of the box already configured from Keystone or it can be treated like any other Express Middleware.

With Keystone it is easy to set up an email management system for an application. It has template-based emails and it's also integrated with Mandrill (Mailchimp's transaction email sending service)

Keystone comes with an auto-generated Admin UI, which makes things very easy for any task that can be completed using Keystone. In any way Keystone is used, the Admin interfaces saves a lot of time and makes any job easier.

Using the data models defined by the developer, Keystone can validate forms automatically without any more setup. Form validation doesn't get easier than this.