I just dusted off some of my old math (hopefully someone younger and more updated can correct me if I'm totally out in the blue).
I think the frequent rebalances of the game is very much a result of the shear amount of possible combinations. It is completely impossible to test and discover all overpowered combinations possible. It even took months for the highly competative PvP community to discover many of the highly efficient builds used today. Some very good players with encyclopedical minds for skills may find most of the obvious game breaking combinations, but far from all! So we have to live with occasional game rebalancing when certain "overpowered" skill combinations start to have a too great impact on the environment (especially in PvP).
Down to numbers:
Say we have 8 proffesions with 100 skills each, to make it simple.
Each primary proffesion then have a total of 7 sets of 200 skills to build a skillbar from.
A skill bar can then have 7x200x199x.....x193 combinations,but since the order in the skillbar do not matter this it is slightly reduced with a factor of 40000. This gives about 400.000.000.000.000 possible different skillbars. The vast majority those combinations are of course absolutely unplayable.
If you then build an 8 ppl team you will get this giant number to the power of 8 and end up with the number of party combinations being a number with 117 digits! I guess that starts to get close to the number of atoms in the universe.
I have now only taken skills into account, add to this armour, weapons and attribute points and the possibilities will increase a lot more!
If now a new expansion adds 2 new proffesions and say 20 skills per proffesion (120 skills/proffesion to make it simple). Then the number of possible combination would increase fivefold. So every expansion will add a considerable complexity to the game.
Lets say I am very happy we do not get random generated skillbars.
(my math might be a bit rusty, but anyway - the amount of possible skill combinations are much, much greater than most people think)