What kind of conditioner you want depends on what kind of hair you have. Cosmetics chemistry is an interest of mine so I've done a lot of research in order to be able to read a label and actually understand what it means. Wikipedia actually has some great information on cosmetic chemistry, but I'm also lucky in that my SO is a chemical engineering student and so we have a
Merck Index.
Sodium laureth sulfate shouldn't be in any conditioner, really, because it's a detergent and surfactant that foams a lot. It is usually in body washes, soaps, and shampoos because of its foaming properties. Some people are sensitive to sulfates; it is a known irritant. That is why many people have switched away from it. It doesn't damage hair, but it may give you a rash if you are sensitive to it.
Typical conditioning agents in shampoos include fatty alcohols, such as cetyl alcohol and stearyl alcohol, silicone compounds and siloxane compounds such as dimethicone and cyclopentasiloxane, and natural oils, such as coconut oil or jojoba seed oil. Some conditioners may have petroleum-based oils such as mineral oil in their formulation. There may also be several "natural" extracts added to the conditioner. My Paul Mitchell conditioner has sunflower extract, white ginger extract, aloe, algae extract, rosemary extract, etc. Panthenol is another common additive to conditioner; it is a provitamin of B5. Obviously, there are more conditioning agents than this, and they can be split into groups based on what they do (are they a humectant, a emollient, a moisturizer?).
If you have dry, frizzy hair, you should look for a conditioner with panthenol, fatty alcohols, and silicones/siloxanes. The higher up an item is on the ingredients list, the more of it there is in the formulation. If you use any kind of heat on your hair in styling it, such as a blow dryer or flat iron, you should avoid choosing a conditioner with natural oils. Think about what we use hot oil for elsewhere--it will fry your hair. However, for hair in need of lots of moisture that isn't going to be heatstyled, natural oils are awesome.
If you have thin hair, absolutely choose a conditioner with panthenol, which will plump up the hair shaft, and avoid choosing a conditioner with heavy natural oils. A light leave-in conditioner such as Infusium may be sufficient for someone with thin hair.
Many people can get away with not using conditioner at all.