You might have come to a problem, when you had to send an e-mail with Workbox and it was, well .... ugly.
Your only option to make it look better was to use HTML with styles and format entire message in the not-so-convenient textbox. We do not have any good Silverlight control that would allow you to create nice looking messages in the Workbox, but we have a workaround.
The workaround is to create a separate list, where you can use the SharePoint richtext box to define the message. For example - have a list with 2 fields - one would be the title, which would define what kind of message this is and another is a multi-line textbox field with rich content enabled. Then inside the designer just use lookup to make the workflow use this field value in the e-mail. This has also a great advantage that you can change e-mail message without redeploying workflow.
How about using lookups in such message?
You have two options - either you combine the message from few elements and insert lookup between them or you can use the "Replace multiple words" to replace a chosen tags with lookup values.
One thing to remember - text in Sharepoint textboxes is using SharePoint styles. Make sure to choose the font family and size to get the same styles in your message.