Attachment options -- the comb attachments that come with your clippers play a huge role in what they are capable of doing. Four attachments is a good number to look for. It allows for varying lengths and cutting styles and will ultimately mean your hair clippers can do everything your expect them to.
Size -- You want hair clippers that are large enough to have a powerful and reliable motor, but not so large that they are too heavy. Good clippers are small enough to travel with, and keep in a case in your drawer.
Versatility: Hair Texture -- Everyone's hair is a bit different, so it is important to have hair clippers that can accommodate different textures.