>It's got a relatively high surface gravity.

Venus, Mars, Mercury, Io, our Moon, Ganymede, Titan and Europa all have higher.

>It is outside Jupiter's massive radiation belt

As are Venus, Mars, Mercury, our Moon

>There is water ice easily available on the surface

As there is at Mars and some would argue on certain parts of the Moon

>Close proximity to the asteroid belt

So is Mars.

>and the trojan asteroids around Jupiter,

They're actually a long way from Jupiter.

>The Jovian moon Callisto is the best body in the solar system to consider for future human colonization.

For what purpose?