On Gamsutra, Jamie Madigan describes two forms of skill trees that are popular in many games. Titles like Darksiders, Mass Affect, Final Fantasy, and countless others utilize either sequential and simultaneous conditioned skill trees that allow players to season their characters to their liking.
The use of skill trees is mainly to give players choices to improve their character so that they can perform to their expectations. As Madigan describes in his blog, simultaneous conditions are skill trees that show everything the character is capable of over time and gives the player a clear understanding of the possibilities. Sequential conditions show only parts of the skill tree that shows the basic skills (of which are very limited) and doesn't reveal the powers progress until the chosen ability levels up. The question Madigan poses is which method is more satisfying for players?
In his blog the studies showed that simultaneous skill trees were more popular than sequential skill trees. Personally, I would agree with the studies because I preferred to know what I was investing in when it came to my character's abilities. In Darksiders II the skill tree provides information on powers that the main character, Death, can learn and improve. I like this aspect because I can tailor Death to my liking and I can judge what powers to use my points on. It is also very useful for making characters ready to deal with specific situations. For example, if I know I'm going up against an opponent that is strong, has extensive health, and is weak against a type of magic, I can level up Death to have health stealing attacks that also deal a specific type of magic damage.
The argument can be made that the sequential method provides a unique hook for players that requires them to continue playing to see all the benefits of their choices, but that can also backfire. Many players may be too impatient to invest in such skill sets. There is also the question of replay value in games that use the skill tree system. For the sequential games, the need for replay to experience all the possible abilities can be a double edged sword. Players might be willing to try another path, while some might find the first experience unsatisfying and might not wish to try another path.