Price Tag: The cost of the unit alone;

Single- or Multi-Cat: Whether the unit is ideal for single- or multi-cat households;

Ease of Setup: How easy or hard it is to set up the unit;

Noise Level: The amount of noise the unit produces;

Maintenance: The level of effort needed to clean and maintain the unit;

Waste Disposal Frequency: The frequency with which you'll need to empty the litter compartment;

Odor Control: How well the unit does at minimizing odors from waste;

Litter Scatter Control: How well the unit does at preventing litter from getting outside the box;

Size: The dimensions of the unit in terms of length, width and height;

Primary Power Source;

Backup-up Power Source; 

Special Placement Considerations: Can the unit stand on its own? Or does it need to be placed near an AC outlet, etc.?