Folklore.org has the history of Bill Atkinson's creation of the program, including the "marching ants" style of showing area selection. The Computer History Museum also has a page about its development including a shot of its predecessor (it was based on a Lisa program called LisaSketch or SketchPad) before the toolboxes were added.
One lovely detail of the program is starting off with the big wide paintbrush selected-- it is so much more engaging than skritching out individual pixels with the pencil tool. (It also probably influenced the choice to use "hello." written in cursive for early promotional shots.) That kind of consideration to first user experience goes along way (especially with something as innovative of the Macintosh, which for many people was the first time they would have tried a computer mouse.)
Another fun thing is Painting and Filling with patterns. Once color came on the scene, these patterns sort of fell by the wayside, but there is something satisfying about seeing the swatch of area filled with a pattern that is fixed independent of the line used to draw it, almost like one is brushing a path of snow from a windshield and then seeing what is underneath.
I still haven't found a modern-day paint program with powerful features and an easy interface for OSX... Paint.NET for Windows was excellent in this regard. "Pixelmator" comes close, though has several annoyances (some of which I just found out were configurable settings, but overall it aims too high, UI-wise, trying to be Photoshop-Lite rather than MacPaint-Pro)