Couldn't I just buy a stylus? Just looking on Best Buy's website they have quite a few for $10 or under.
You could, but it would be a totally different and completely inferior hardware implementation.
The Note tabs have a wacom digitizer built in I believe, so their fine point stylus can detect various pen strokes and pressure sensitivity. All that while still allowing you to rest your palm on the display while using it without causing any interruption (your palm won't create it's own pen strokes where it rests).
This doesn't even take into consideration the incredible bundled software that's integrated well.into the OS (Un-sheath the pen causes a blank page to instantly appear for instance).