Here's some numbers for you, guys.
Earth's moon has a 1738 km radius, making it a little over one-fourth the width of the earth. It's a rocky moon with a high density (around 3300 kg/m^3), giving it a surface gravity of about 1/6th G.
Ganymede (Jupiter), Callisto (Jupiter), and Titan (Saturn) are all icy moons, meaning that their average density is rather low, around 1500 kg/m^3. They're all around the same size, about 2600 km in radius, making them about 40% the width of Earth. Since they have approximately the same density and size, it's not surprising they have similar gravities, all about 1/7th of G. Note that although these moons have similar stats, they're pretty different otherwise; Titan has a methane atmosphere, and Europa may be liquid water under an icy shell.
While we're on Jupiter, let's not forget Io. This is a rocky moon about the same size as Earth's moon, but a little denser. Io's gravity is about 1/5.5 ths of G. Of course, Io being a *molten* rocky moon studded with volcanoes doesn't make it very habitable.
Finally we have Triton, the true gem of our solar system as far as moons go. Triton is about the same size as Earth's Moon, but is very dense, making it a rocky moon, although its surface is covered with ice. Triton's gravity is 0.29 G -- which sounds pretty comfortable. Unfortunately, at a surface temperature of 38 K (-235 Celsius, or -391 F), its atmosphere is mostly hydrogen and if it rains at all, it's raining liquid helium. So I'd have to say its planetary conditions are 'Unpleasant'.
The distinguishing feature between icy and rocky worlds seems to be their density, which has direct effects on their gravity, surface structure, and magnetic field. Icy moons are low gravity with no mag field. Rocky moons are higher gravity, with a mag field possible.
All data taken or calculated from the tables in my astrophysics book. If you find I've made a mistake somewhere, let me know!
[This message has been edited by Shepherd (edited 01 March 2001).]