Nobody's mentioned this, maybe everybody thinks its a given...shooting digital. No film to buy, no wait for processing, processing totally under your control if you want, exif data at your finger tips, etc etc etc.
I guess the second best feature of my DS is the backwards (and in some instances the forwards) compatibility of lenses.
Runners up:
Standard batteries; small size; pentaprism; SD cards; and good quality primes available.
NaCl(stupid camera still doesn't give me a chocolat truffle when I nail a shot tho!)H2O +