In Nintex 2013, requirement was to show the user's "Display name" in email. looks a very simple issue by looking at the definition. However, to achieve this in workflow have to choose proper way to implement.
Two options are available to get this.
1. Using "Call web service" action
2. Using " Query user profile"
To move ahead with first step, we need to use Regular expression, web service and query xml as described here in https://community.nintex.com/thread/1353 lots of configuration require to make this work. as well to store credentials of admin to call the web service, need to make the constant in Nintex management.
So In my implementation, I choose second option, fairly simple to use.
Steps to use "Query user profile" action to get display name:
1. Add "Query user Profile" from actions tab
3. Expand the properties to retrieve, and select "Name" , click on Add
5. Now, add the email notification action and from insert reference add the workflow variable.
6. That's all we are done.
No configuration/web service call/credentials require to store.
Hope this will help you!