more realistic:
-the cars dont blow when ya shoot at the fuel tank, instead, use tracer bullets
-as everyone said, bullet holes, in people and buldings
-better sounds
-more gore
-destroyable buldings, like if ya crash,and then it gets restored
-Avtomat Kalashnikova 1947, with realistic sounds, and much better than in GTAIII
-that the mags go off weapons when ya reload
and for last, that when people gets shot, like in the chest, they fall, and start crawling and complain