The leveling experience was amazing. Lots of voice-acting, highly-streamlined quest progression, and an awesome soundtrack = great questing. However, the best part, at least to me, was that zones now scale with character level; players can pick and choose which zone (story) they would like to play through first, without running the risk of out-leveling the story. I can't tell you how many times I've had to skip leveling in certain zones, or missed the epic finales of various quest lines, due to leveling faster than quest-efficiency.
As for the artifact/class hall implementation, I main an Arms/Fury warrior; it's been great so far. Legion Arms is very active, and always has meaningful buttons to press in its rotation, and feels much healthier than WoD Arms. I haven't played much Fury yet other than prepatch, so I can't speak to how it's performing.
In WoD, I raided as a dps warrior in a guild that had M Archi on farm, and had some warrior parses on Warcraft Logs that were as high as #12 in North America (out of thousands of other warriors); in past expansions, I've been primarily a PvP main, and have hit High Warlord/Hero of the Horde on my warrior, though I've only been ~2200 in arena (on both warrior and warlock). I can say without a doubt that this is the most I've ever enjoyed playing my Arms warrior (without set bonuses in the picture).