Size of the pins/blocks
There's a basic rule of thumb that says:"The softer the dirt, the bigger the pins need to be.". Long pins work by penetrating the (soft) soil, and short pins usually work by scraping off the upper layer of the soil.
Bear in mind that long pins and very soft compounds don't mix very well, then the spikes just bend over instead of penetrating the dirt.
Density of the pins/blocks
For a given tire width, the density of the pins is inversely proportional to the weight supported by each pin. There's usually an optimum, where the tire works best. For example: if a certain tire works very well when it's heavily loaded, but doesn't feel right when it isn't loaded, the tread pattern is probably too dense. This can happen in very dusty or soft conditions.
Tires for sticky mud usually have a very low spike density, because too much spikes cause too much stiction for the amount of traction they create, slowing the car down.
Pins or blocks
Round pins provide grip that feels the same in all directions, it feels consistent and it's somewhat easier to slide. Very useful on difficult, low-traction, bumpy tracks.
Square blocks feel more 'edgy', they can give the impression of generating slightly more grip, especially on smooth, hard surfaces. The forward traction they generate also feels nicer.
Center tread
Some tires have a larger tread pattern in the center, often shaped like an X, an I, or a T. In all cases, it increases forward traction dramatically.
Note that on 4WD cars, more forward traction from the front tires can also mean more steering.