Giancarlo Stanton is still just 24 years old. Which is why many believe he's yet to hit his ceiling.
After hitting 37 home runs in 123 games in 2012 and leading the National League in slugging percentage, Stanton missed some time again in 2013 and hit 24 while batting three more times. He also drew 28 more walks -- perhaps because of the historically inept lineup around him in Miami, perhaps because he just became more discerning at the plate.
Jayson Stark has a story on Stanton and how often the Marlins slugger may get pitched around this season. Jayson quotes a scout saying, "I also think he might walk 200 times." That's a bit of hyperbole (I hope); only Barry Bonds has walked 200 times in a season and only five players have walked as many as 150 times and 150 walks would be doubling Stanton's total from last year.
But you get the point: Stanton's home run total will be affected by how many pitches he does get to hit. As well as his ability to stay on the field. The projection systems see Stanton playing 135 games or so and hitting 33 to 36 home runs. That's no fun. Let's assume good health and put the over/under at an optimistic 39.5.