Here's my thoughts.
1) From a usability point, I can see it being a major advantage. Click to edit, have the operation screen pop-up with the existing parameters, edit what you need to, click a button, then the g-code file gets updated with the new code.
From a programming point, in order to keep everything contained within the g-code file, it's quite involved with parsing/checking/updating.
2) I'd quite like some form of database, so you can create a basic library of previously used speeds/feeds for each tool and material. Select a tool for an operation, then up pops a list of previously used speeds/feeds for various materials. However this does have the potential to lead to a lot of information either of little use, or hard to present in a user friendly way. Another possibility is the addition of a basic speeds/feeds calculator.
3) This although sounds good, but would it actually be used? I'm struggling to think of a realistic scenario where you would actually want to re-order operations..
4) I have my thoughts, and I know what Tom's thoughts are, but am interested in other peoples'.