After reading alot i have noticed alot of eq1 players are really into Warrior Cleric Bard classes from these being overpowered in a different game.
I hope that the unexpected classes get to shine and be needed in every area of the game.
When i read and hear about only certain classes being able to do content it really seems flavor of the month.
If hardcore groups are going to switch to flavor of the month every update that will also mean they do not spend enough time becoming the best they can be at a specific class.
A great player can push the worst class to be great.
I completely understand your concerns but I don't think that will be an issue in PRotF. If the difficulty of leveling is anything similar to EQ1, I don't think that PRotF will fall into the FotM (Flavor of the Month) cycles like WoW. Players may have an alternate character (or maybe two) but the devs switching power from class to class to force players to continue playing to pay subscriptions (FotM) won't likely be a tactic used in this game.
edit: This is not to say that there won't be adjustments made to classes, but the adjustments won't be made in the interest of making a class more powerful than others "intentionally".