Fair enough everyone has their opinion! I'll go though each point;
1.) Well I see a class as a person who has a set job, this may be as a healer or tank as an example. But this is the part I don't like about it, why should I have to pick between being a tank or a healer? Why can't I be for example a class that is 66% tank and 33% healer. Because of this, a server wide skill tree would be a great solution. (On another note, because it's server wide. People can't complain about having to be a certain class to obtain a certain skill tree. This little note should cover part of point 5)
2.) It was only an example, I know little about actual terms for class's like that
3.) You may see this as a solution to the best way about it but adding new skills and that would be a lot harder compared to a single main tree. A problem that may come up in separate tress could be not being sure what class's skill tree to add it too and other problems like that (balancing issues eh)
4.) right this is the part I'm trying to get across, there WONT be balancing issues as long as the SPECIFIC skill is balanced! an example of this would be a fireball. It's damage would be 100 with a cool down of 7 seconds. This is a balanced skill, but when combined with another skill (I don't know, a skill that increases magic damage for x amount of time) it will be more powerful. Because of this, players will structure their skill choices a lot more carefully
5.) Yes sure you're giving players more options within classes currently but why not give players access to all skills down certain paths meaning options and combos are unlimited. Also see point 1.) in the note at the end!
Hope this covers everything