It's fine to shoot with the exposure on Auto, but there are times when that results in subject brightness that is different from what you would like it to be. At times like that, try using exposure compensation to bring the shot a bit closer to what you had in mind. If you go all out with around 1 stop of compensation, you'll be able to see the effect it has. The fact that you can check the image immediately on the LCD viewfinder on the back of the camera is one of the advantages you get with a digital camera.
I shoot nightscapes at low ISOs whenever possible, so to prevent camera shake I use a tripod and a cable switch. Still, in situations like when it is dark, the subject is moving, or I want to hand-hold the shot, I will increase the ISO speed to bring the shutter speed closer to what I want it to be. Of course, you can also get more brightness by opening up the aperture as well.
Compared to daytime shots, the shutter speed is slower for nightscape pictures, but by lengthening/shortening the amount of time the shutter is open, the expression changes even with the same subject. This is particularly easy to see with subjects that are moving, so try changing the shutter speed to match your intended expression. In such cases, by changing the aperture value and ISO speed, you can take pictures with the same brightness even if the shutter speed changes.