What to look for in buying laminate flooring
Great question! My wife and I were examining laminate flooring a little while ago too. Not all of them are alike -- here are some things to consider.  


Check the warranty. Cheaper laminate will usually have a shorter warranty with more conditions (e.g. 10 - 15 years only protecting factory defects).  More expensive laminate will usually cover more conditions for a longer time (e.g. 25 years+ covering factory defect, normal wear and tear, fading, staining, denting and water resistance).  Keep in mind that these are examples -- every manufacturer is different.  
Find out details of how a sample is made A lot of laminate flooring is now treated with Aluminum Oxide which protects it against scratching and denting. More expensive laminate may have more coatings.  If you're expecting heavy foot traffic or have animals with nails, you might want to take it into consideration.  For answers to questions like this, you might need to contact the manufacturer directly. 
Thickness varies Laminate thickness can vary anywhere from 5mm to 12mm.  The thicker laminate can feel more stable and supposedly be less likely to have problems with humidity changes. 
Joint integrity varies Not all laminate flooring interconnects in exactly the same  way.  Some joint connections can withstand weight and water leaks better than others.  Name-brand manufacturers claim to have more experience with this. Be sure to check how the pieces fit together.   


-M