It's the 3rd best selling game of all time for a reason!
The gameplay is better than ever, as I'm sure you know. The driving feels faster, probably because the game gives you great cars right off the bat. There are now motorcycles, which are definatly a welcome addition to the game. Shooting is slightly improved too, with a better weapon selection and an improved targeting system.
There are still loads of problems, however. Even though the game improved its targeting system, it's still terrible. It seems like it would rather mow down civilians than the thug shooting at you. Which it does, frequently. This isn't a major problem, seeing as how there are weapons with crosshairs, and you aren't required to shoot people on the street very much. The A.I. is awful as always, and sometimes run straight into moving cars, and this can be a huge problem during missions with A.I. partners. I can't think of a single mission with A.I. partners that I failed from my own fault.
Damage doesn't make much sense, I can be shot in the face and lose about 30 health or so, but if I fall off of my bike while on a wheelie, I lose about 90.
The story has improved A LOT. The main character actually talks and has a personality, unlike the previous games. His name is Tommy Vercetty, and he is one awesome dude. The best way to describe him would be badass. TOTAL BADASS.
If you haven't picked this awesome game up yet, do it. NOW.