On THJ, that was what was so awesome... You could find combinations that would surprise you, pop in and out of "the best" meta, and it would evolve with every new expansion release as new items, and spells unlocked. Most of the AA's were available out of the gates, but they would do balance tweaks quite often.
Pet cucks (Multi Pet classes) were always good enough to do most content and were the easiest. Plate Tank + proc's for survivability were always popular. Monks were a popular classes to include in your melee build because they got triple attack & good avoidance. Then you had some interesting ones like Enchanter come out of nowhere because they had Dopplegangers that would create copies of your character and cast an insane amount of DD's or stacking DoTs while they would tank for you.
My most successful character was a Monk, Cleric, Berserker. Monk and Berserker gave me crazy damage, the ability to feign-flop right to where I needed to go to complete content farm quickly. Cleric would give me spot heals, Cure's (very important), the ability to wear Platemail, and this AA called Divine Avatar on a 10 minute timer. It made your character so broken, they had to nerf it a number of times.
With something like 560 combinations, sure there were some that felt weaker than others, but most of the time you would find that any class would add meaningful and real power to your build. The real powerful combinations came in the form of obscure AA's that could be combined or stacked with other AA's to do some really wild stuff. There wasn't much value in classes that did things like Haste, SoW, or other fundamental stuff like that because they had world buffs that provided those buffs for you. So the developers gave relevance to classes like Shaman's by making DoTs instacast. Even one dimensional classes like Wizard were great because they had AA's that would let you get a higher Crit Nuke % than any other class and it applied to Proc's which were a huge thing for survivability and damage.
It might sound silly or overpowered from the outside looking in, but they put so much thought into the balance of things, it all really meshed well together. I am not sure I would trust this type of server in the hands of someone other than the THJ guys. The server really was brilliant, and it showed in the popularity.