As for the design direction, from vanilla to the cata prepatch, there was the original talent template, with a few changes occurring here and there sometimes.
It was also in the ICC patch the dungeon finder (and subsequently automated teleportation straight into dungeons) came, and this patch brought a large jump in stats as well. I actually quit playing on my original account at this time, because I felt epics were given too readily in the new dungeons and the stats jump made it feel very overpowered.
With cata however, since the system worked in the way of providing talent points with levels, they felt people had too many points. So they reduced the amount of points in cata so it would only fill up a little more than one tree, while in wotlk there was enough points to fill much more than that.
So they compensated for that change by implementing a few new abilities and passive abilities, however they limited the abilities/spells to be specc-based at this point if memory serves me right. It was either then or with MoP, but I’ll presume it was with cata.
Then they replaced the entire talent template to the template we still have today, with the MoP prepatch. To compensate for this big change, they implemented many new abilities and passive abilities (passive meaning no need to press, they’re always active and some had proccs etc.).
But then in WoD, they felt they had gone too far and thus “the pruning” happened. While they did remove a lot and changed things, they also provided some more passives and an ability or two to compensate.
That’s what happened. But the REASON for why this happened, is that Blizzard first designed the encounters to fit the classes. So class designs took precedence in the design process, and encounters were made to adapt. But then they decided to reverse this flow of the design process, and started to design more impressive encounters but then had to make classes fit the encounters. This led to a simplification of class designs for the sake of empowering the encounters.
While I agree with that design style for PvE, that encounters are supposed to be the challenge, and less focus on the abilities of the classes, it’s the direct opposite of what PvP needs.
PvP needs room for skill differentiation, risk/reward structures, and a high climb for skill progression so the challenge never fades into feeling monotone.