Sandman
Depends on what you use in it. All layer effects and layer adjustments are fully reversible. I'd say that these days, most of your ordinary photo processing in Photoshop is reversible by default.

nospam
you have to make adjustment layers or smart objects for everything you do, otherwise it's destructive.

it's an extra step.

Sandman
I.e. if you open a photo in Photoshop and click the Levels button in the palette, which is the easiest way to apply a levels adjustment, it's fully reversible. Only if you select Image ->Adjustments ->Levels do you get a levels adjustments that isn't reversible.

nospam
the easiest way to do levels is pick levels in the adjustments menu which is not reversible.

Sandman
The filters menu is non-reversible unless you take steps beforehand to make them reversible (i.e. enable smart filters).

nospam
that's my point.

iphoto is not non-destructive. it makes a copy of an image when you change it and writes the changes to the copy.

Sandman
I.e. exactly like Lightroom. LR has a better UI for enabling and disabling effects, but the process is the same.

nospam
iphoto is nothing at all like lightroom, other than casually in that they manage assets.

all you can do is revert to original.

Sandman
Or use the sliders in the other direction, same result. All adjustments can be reversed individually in iPhoto. Well, all except retouch and red-eye I think.

Also, the "revert to original" changes to "revert to previous" depending on what you're doing, so some edits can be reverted step by step.

nospam
that's an undo.

if you quit, it's not available.

Sandman
It's not as sophisticated as LR or Aperture, of course, but it is 100% non-destructive.

nospam
it's not even close to either.