Okay, THAT trailer makes me a lot more confident about the game. It's not necessarily the graphics or the environment (although they're both cool as shit), it's the characterisations. The protagonists and NPCs seem more interesting than in IV - not that I hated Niko, but most of the other characters were sub-par. This time we have cultural and generational gaps that look as though they'll provide a lot of depth and some laughs (something sorely lacking from IV). As KJ said too, it's more action movie-like and not so 'gritty realism' kinda thing - like the original SA, swinging on boat masts, jumping off crashing trains, etc. This is what I like to see in an action game.
"Bounce? We're bouncing now?" - awesome stuff.