The mini-forms and views (if you can call them views, they are more like preview forms as they are not displayed as a grid) that are used for creating and viewing Activities in the Conversation Tab (the tab that includes Posts, Activities and Notes) are not customizable, which presents a significant issue for any customer with mature activity management processes.
These mini-forms are not even available in the list of forms for the entity - they are apparently NOT the Quick Create forms.
Adding fields to these forms is, of course, a balancing act as keeping the number of fields to the minimum in this area is crucial for keeping the screen very clean and easy to work with. Having said that, it actually often comes down to just 1-2 fields that you need to add, yet they are critical fields to have, so you cannot use the area without them.

To make this area more usable sooner rather than later, perhaps this can be addressed in several stages:
1. Add a System-wide or, preferably, a personal setting for each Activity Entity to choose if the full form or the hard-coded mini-form should be used when creating Activities from the Conversation Tab. Email, Appointment and custom activity types are already using the full forms in this area, so hopefully this is a simple change that can be implemented quickly.
2. Make the ConversationTab-CreateForm (mini-form) editable for all Activity entities.
3. Make the ConversationTab-PreviewForm editable for all Activity entities. This might be more challenging technically if that preview is using the activity pointer table, however perhaps it is time to address the limitation of not being able to include custom fields in activity views, which is a problem as old as Dynamics CRM itself. We have been writing code or using third parties to address this since CRM 1.0, we are still writing code for this 10 years later for CRM 2015... for such a basic feature... Customers are becoming a lot smarter about leveraging CRM activity management functionality, this is coming up more and more.

So many exciting new features in CRM 2013 and 2015, hopefully this area can be improved so we can use it to its full potential.