While syntax highlighting diffs in general would be difficult, it would be easy (and super useful) to apply it to new files.
(All I did to create the mockup below was to take the regular file view and apply background #DDFFDD. It could be that easy.)
In beta 11, Opera’s going to hide all “http(s)://”, and also all querystrings (until you focus the addressbar). Opera’s devs are right that users consider them mostly “gibberish”, but I think this change could cause a ton of problems and confusion for people, especially support staff, and there are plenty of sites/apps still out there with URLs based on querystrings. I can see this setting as short-lived.