I agree with a lot of what has been said about what helps slow speed performance, but would like to add more...
In most if not all cases, the higher the gear ratio the better. How high is subjective. Some modelers like the option of being able to run their locomotives at higher than scale speeds. Others want the max motor speed to correspond to the max speed of the prototype modeled.
A reasonable gear ratio (IMO, between 25:1 and 50:1 depending upon drive wheel diameter and motor size), with a smooth, bind free chassis, decent motor and good controller, will produce excellent slow speed running. I've built chassis' with scale 33" diameter drive wheels, a 36:1 gear ratio and a Mashima 1020 can motor that will run smoothly and consistantly at under a scale mph.
Really excessive gear ratios (some mentioned here were above 200:1), are fine if you like mind numbing slow speeds, but require that the motor be turning pretty high rpms for anything but a snail's crawl. This can contribute significantly to noise (but necessarily always), motor and gear train wear. Through experience, I've come to the conclusion that anything above 100-120:1 is not practical for small drivered Narrow gauge locomotives. But to each his own...
Better motor quality and larger motors can make up for lack of gear reduction, as does a good controller. High quality Coreless motors make up for a lot of short comings, but can't work miracles on a poor quality chassis. Same can be said of DCC, or a good DC controller. There's only so much you can do for a bad running chassis.
One of the biggest benefits to operation, at least from my experience is adding compensation/equalization to a chassis. In instances where the modeler is limited to off the shelf commercial chassis', then what was said above applies.
This is my first post at the GnATTERbox, and I look forward to being a part this group.