Q. What is skill tree?
A. The "skill tree" is eventually a concept of ability accumulating system that can be traversed differently by characters and choices, and is not necessarily a tree.
Q. Why skill tree?
A. It is one of the forms of customization a RPG should provide. Without customization, an ARPG is eventually an Action game and is why some felt Diablo 3 lacked something important comparing to its prequels - because it has near to none RPG elements. (If I wanted to play an action game, DMC and GoW are definitely better, you know what I mean, there are too many better choices.)
Q: Why full respec or skill pool is bad?
A: Full respec means you can always choose the best leveling build, ignoring the risks of making bad decisions that fail your characters midway. Did you ever feel the first 3 difficulties are boring and unnecessary? It is because they only serve the purpose of unlocking skills in the pool. A better model is you make decisions to accumulate your power, then test them in well tuned difficulties to see the pros and cons, and fix your build without falling off the final ideal character. If you make enough bad choices to stuck in Nightmare, then it is your own fault. Otherwise, we don't need so many difficulties, one is enough, we unlock everything without choices anyways.
Q: But isn't it stupid to reroll just for trying a new skill?
A: Reroll is not as bad as you think when you can have fun in the process instead of rushing from 0 to 60 mindlessly.
This is not a troll post. Anunnaki#3684 is my profile.
Q: After some while, all superior builds will be created, players will just copy from the Internet, if the tree and decisions will finally become redundant, why should there be a tree?
A: What you eat will finally become poops, then why do you eat tasty foods? Because even eating for nutrition can be entertaining, not to mention we are playing a game. To be realistic, Diablo 3 is not a subscription based game that is meant to be expanded and played forever. It will go to an end like every other game, what really matters is whether the players quit with smiles on their faces and whether they will pre-order your next title for the good old experiences and respect, like what we had in other Blizzard games.