The first advantage of course, is the size and weight, so you get a better workout and build more muscles.
Larger sensor - for less noise and greater control over depth of field
Manual zoom lenses - for precise zoom control
Minimal shutter delay - as little as 50 mS from pressing the button to taking the photo
Interchangeable lenses - one size does not fit all, allows choosing the best lens for the subject
Viewing through the lens - for precisely lining up the subject & background
RAW - up to 4096 times as much information in the image, greater color depth, greater image control
External flash capability - more range, bounce flash
You don't *have to* learn all about ISO, aperture and shutter speed to use the SLR but to get the greatest benefit it is well worthwhile (as it is on a P&S also).
I like P&S (have 2 of them) but to me, the P&S are like an electronic thingy that takes pictures, the dSLR is more like a camera that doesn't use film.