Quick Reply is made as a "Quick Response" block, so you can make a response to a post (or thread) before you forget or as you read them. Quick Reply is meant as a "no thrills" response box, as it's just there for a fast/non-formatted response. Reply however brings you to a "Full Response" page that allows for heavy posting. You can also click the "Full Reply" button from the Quick Reply box which will forward you (and all of your Quick Reply text) to the Full Reply page.