When creating and selecting a workflow command now you will notice there are three new fields. Comment Template, Comment Dialog Height and Appearance Evaluator Type (more about this later in another blog).
I created a template to be used for the comments. I kept it simple, but you can do a lot of different fields custom to your site.
I set the command with the following values. Keep in mind that you could have different comment boxes for different commands.
When the command was clicked the following popped up. Simple and nice!
The workflow is such a great and important feature in Sitecore. I am so glad it has been updated in Sitecore 8+. It just goes to show you that Sitecore listens and is always improving their products.