it used to be that u get new talents and abilities, so no ur power doesn't get reset
in TBC u learned 5 new spells
in wrath some classes 2 and others 3, and i still remember ppl mocking blizz giving less abilities than tbc and they had to defend by making a far more complex talent tree (which they did, and it gave them a nightmare to balance in entire exp, if u remember 50/50 comb/sub rogue built with infinity combo points, healadin with protection built, etc)
then came cata and they started to regress ur power by screwing talents, then wod finished it off with pruning even abilities u learn
So no, for at least at start u used to grow in power every exp, now u get weaker because they are too lazy to work, as they even flat out admitted during BFA that they aren't really working on it (which caused a big pita for PR and they had to retract that word later and say it was out of context etc)