How easy is it to clean - Your outdoor and patio furniture is exposed to more dust and dirt than indoor furniture. Therefore, it is important to choose furniture with few cracks and crevices for dirt to accumulate. Look for furniture with lots of smooth surfaces that are easy to wipe off with a wet cloth. Some grooves and designs are fine if they are are big enough to wipe down with your finger.
Durability - The air in a patio or porch can get much more humid in the summertime than an indoor air conditioned environment. Water in the air will make your patio furniture wet exposing it to potential rot. Choosing plastic or sealed wood is a great choice, but be wary of cheap unsealed furniture that will degrade easily.
Sunlight - Ultraviolet rays will degrade many materials, so you must look for furniture designed to be UV resistant. Many plastics, fabrics, and colors like blue are more prone to fading when exposed to sunlight for long periods. If your furniture is in your patio or on a porch with a roof this isn't so much an issue.
Height - Look at how your patio is designed and what other furniture is already in it. Many people have a bar in their patio, and they are looking for bar height furniture with is usually taller than regular furniture.
Design - You need to decide what theme of furniture you want in the patio. Most patio furniture is made from wicker, iron, or wood. Make sure you are sure what material you want before you order, and in the future you won't have to deal with furniture that dosn't match. Wood often changes color with time, and bright wood colors on new furniture pieces may become more of a light gray after they sit in your patio for a long time.
Bar Height Stools and Chairs - Consider whether iron or wood is a better choice for your bar. Do you want stools with no back or actual bar height chairs with a back rest? Simple stools are going to be slightly cheaper, but if you plan on sitting at the bar a lot you may want to go with the more comfortable chairs that have a back.