Fangraphs has a great series using ZiPs, Fan's Fielding Rating, and their own projected playing time to project out WAR for each position for each team in the majors. I was going to do a summary as to where the Giants' ranked, but I don't need to. The Good Phight has done it for me. For pitchers, I had to do it myself, and then I combined position players and pitchers.
The Giants' position players are expected to post 24.5 WAR, an improvement of 6.1 from 2011. They'd be the fourth most improved team in the majors. That would also rank their position players as 9th in the NL. Last year they were 15th in the NL (the Pirate were worse).
The Giants' pitchers are expected to post 18 WAR, 4.9 less than their 22.9 WAR of 2011. This would still rank them 2nd in the NL, behind the Phils.
The Giants' total projected WAR of 42.5 is a half game ahead of the D-backs projection of 42. The D-backs produce 4 more wins from their position players. The Giants' pitchers produce 4.5 more wins than the D'backs pitchers do.
There you have it. The Giants win the division by half a game!