As a young child, I found absolute delight in playing with the minifigures, and still do as a teen! Nevertheless, I have always considered both the build AND minifig portions of a set as equally vital aspects of the building (and playing) experience, the minifigures add depth and more life to the build while the build itself offers an exciting environment for the minifigures to populate.
While Lego is clearly aware of the appeal of minifigures (and how many people are willing to buy sets solely for them), they rarely (IMO) truly exploit the fact. What I absolutely love is that with most themes, Lego offer interesting builds (most of the time) along with detailed and well designed minifigures to compliment them, which not only enhances the playing experience, but is a testament to Lego's increasing quality over the years.
However, licensed themes often prove to be an exception to this, this year's Star Wars winter-wave spring to mind. The sets are well designed for the most part, but it's evident the emphasis is on the minifigures themselves - for example the AT-AP commercial ends with pictures of the boxed sets AND the minifigures, which (to my knowledge), hasn't been done before.
The Superheroes line also seems to suffer from this flaw - 2014's Marvel sets are clearly emphasising on minifigures instead of builds, the Hulk Lab Smash especially harkens back to the days of much simpler builds, while still offering superbly designed minifigures.
But time and again licensed themes also offer minifigures and builds which compliment each other well - the Lone Ranger theme is a shining example of this.
I think generally, it's a matter of patience. Kids want to play with what they just bought as soon as they open it, and while the building experience most definitely is fun, it can also be tedious (as most have said already) for the younger age groups, they want something to play with as soon as possible - enter the set's minifigures (though as Aanchir has stated, Lego is beginning to spread out the minifigures across multiple bags for the larger sets to also focus attention on the building aspect).
Minifigures are an extremely important aspect of all System sets, and while Lego certainly puts a lot of effort and detail into their minifigures, they certainly won't tip the scales to favour them over the build, they did start off as a building block company after all, and will never forget these roots, the build makes the set as much as the minifigures do - it's all a matter of being patient with the builds for the younger fans.