There are four things that will determine the quality of sheets. Let's take a moment to look at all four.
First, and this is what most people concentrate on, is thread count. Thread count will tell you how tightly woven the sheet is as long as they are being truthful. There is no standard for thread count, except it is the total amount of threads in a one inch square of the fabric. Some manufactures define thread as all the ply's in the yarn that makes the sheet. If he uses 6 ply yarn, and has 100 yarns then he claims he has 600 thread count sheets. Most people if they looked at the sheet would see 100 thread count. Then there is the physics of space. There is only one square inch, there is a maximum number of threads that will fit. In most cases that is a maximum of just under 600 thread count. That tells us that anyone who has a higher count is fudging. Or maybe worse, using a thinner thread than would be best for the sheet. Any time someone stresses the thread count, they are either lazy, or that is their sheet claim to fame. Buy from better stores, and know that most are competitively priced for the quality of their sheets, in other words you get what you pay for.
Second is the material that makes the sheets. There are cottons, wools, synthetics like polyester and blended. Depending on what you want out of the sheet the type of material is important. Some of the best cottons in the world are grown in Egypt. But not all Egyptian cotton is necessarily the same varietal. Like anything else, there are quality strains and lesser strains. The same cotton grown in the US may be branded as Supima. Supima is a self regulating association that makes sure the cotton is pima quality. Always look for this key word when shopping cotton sheets: Long Staple. The staple is the length of individual fibers, and the longer the fiber the less likely it will pill. The natural fibers like cotton or wool will allow more breathability in the sheets and help make the surface a little less warm. Synthetics will perform the best coming out of the dryer with less wrinkles.
Next is the weave pattern of the fabric. In sheets, the two most popular are sateen and percale. Each weave imparts different qualities into the sheet material. Sateen is the softest weave, but it is also the worse to come out of the dryer. There is something to be said for the feel of the sateen weave! Percale is a stiffer weave, less wrinkles and more crisp feel on the bed. Mostly the weave is just personal preference, one is not better than the other. When you find sheets that feel good to you, make note of the weave.
Lastly, it is the over all feel and durability of the sheets. Assuming we found the best cotton, it is important how we feel as we lay on them. Most of the time, better quality translates into better feel. When you find a brand that feels good, go back to it over and over, unless they disappoint you. Only time will tell durability, and how we maintain the sheets will impact the durability. Most people will wash their sheets about once per week. Sheets should be washed separately from other items to avoid pilling. If you want, an iron can make them nice and smooth. Keep in mind that some weavers treat their fabric with enhancers that will wash out after a few washes.
Hope that helps, to recap the four things to look for in sheets is: Thread Count, Weave pattern, Material, and feel. With the internet there are many resources available from consumer reviews, to the BBB, to consumer organizations. Use them and find a great sheet! That way you will...