Mafia 1 was utterly incredible for its time..just consider, it had graphics on par with Max Payne (which was graphical benchmark and powerhouse back then) but in huge open world city, with traffic and very realistic physics to boot.
Plus it had extremely varied missions (each taking place somewhere else, in unique location - motel, bank, steamboat, prison...), one of the best stories ever put into videogame (not many games have surpassed it to this day..if any), I could go on!
And it was made by 20 people studio as its second game.
Mafia 1 is and always will be a masterpiece.
Plus it ages pretty well. Here are few screens I made when I replayed it few years ago:
http://img18.imageshack.us/img18/7939/game2009100323065223.jpg
http://img406.imageshack.us/img406/1148/game2009100323101320.jpg
http://img255.imageshack.us/img255/2341/game2009100323121809.jpg