It’s off by default. You turn it on via the General prefs panel, where it asks if you want to highlight differences for updated items.
Deleted text is struck through and red; new text is green and underlined.
This is another one of those features that I didn’t think I’d like, but NetNewsWire users asked me for it. Aaron Swartz, in particular, wanted this feature—and he said we could use his script in NetNewsWire. (So if you look inside NetNewsWire’s bundle, in the Resources directory you’ll find htmlDiff.py, Aaron’s Python script for showing HTML differences.)
So, if you like this feature, be sure to thank Aaron.
Now, as I said, I didn’t think that I personally would like this feature—but it turns out that I totally like it.
That leads to a little advice for app developers—your users are smart. The list of features I didn’t think I’d like, but that I did anyway because people asked for, is pretty much the list of cool features in NetNewsWire. (Groups, for one thing. The Combined View is another big one. And so on.)