I enjoyed Mafia 1 for the story, characters, and the setting. I didn't play Mafia 1 for the sandbox as I felt the GTA series did sandboxy'ness well, though going for a drive about in Mafia was very engrossing and engaging in its own right. Mafia 1 had a sandbox but often it felt empty and bereft of impact, there was reaction from the world but only in a limited and halting manner. Speed was enforced, yet not other traffic laws. You could stop for gas, yet never really had to. So much more could have been done to fully realise the sandbox, and since these things were left out I always felt in a sense that the sandbox was an afterthought, some effort was made but the developers were forced to quit the sandbox early, perhaps. The GTA series has always felt the opposite to me, a world to play about in with some story elements. GTAIV was a step towards Mafia 1, I felt, in many regards but most notably in the importance and quality of the story, characters, and background.
From what little I can ascertain from the demo, the Mafia series seems to be trying to take a step towards GTA, in terms of the sandbox and 'emergent gameplay'. With only 10 minutes to wander in free roam, it's difficult to get a coherent sense of what the new game is trying to accomplish for certain, but there is much more of a reaction from the world when you decide to muck about. Mafia 1 felt to me like a game that sacrificed the sandbox fun of the GTA series for a better story, and I was happy with that. I'm keen to see whether Mafia 2 has sacrificed time spent on the story in order to bring about a bigger, better, and more rounded sandbox feel.
I'll likely enjoy the game either way as I am the sort that wanders down the streets and alleys of make believe cities while peering into the windows of buildings I can never enter, marveling at the skyscrapers looming above, and noting the myriad details of the gutters below. The demo alone provides this sort of eye candy in spades.
P.S. The loading times are phenomenal.