So this is kind of a shitpost but I'll indulge you. Just know that there's plenty of dev blogs/roundups on the likes of Wowhead, MMO-Champion, etc. that cover all the new features/changes.
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Legion is shaping up to generally be a vast improvement over Warlords. If you didn't raid in WoD, you probably found the end-game content to be lacking and you'd be right. This is alleviated in Legion with the introduction of few systems, primary one being the world quest system which ensures there's always something actively worth doing that will progress your character in some way (be it artifact power, gold, gear, etc.) when you login. The other main thing for non-raiders is high-end dungeon content in the form of Mythic+ dungeons which function similar to pushing greater rifts in Diablo 3. Mythic+ difficulty increases the higher you advance, mixed with different combinations of affixes each week that throw twists in the gameplay (e.g. one affix is players taking constant damage over time during the dungeon). This is besides the leveling zones being top-notch, along with the 110-only zone of Suramar which has its own expansive story. And of course raids and PvP as usual.
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As for PvP there's a new revamped honor/prestige system (cosmetic rewards for prestiging like in Call of Duty) with PvP only talents/abilities. Gear also now matters very little since in instanced PvP combat players are scaled to a set power level with your iLvl only increasing damage/healing by a marginal percent comparative to previous expansions. Though you can still get gear through PvP, just rewarded from crates for winning BGs and Skirmishes with RBGs and ranked Arena giving higher iLvl stuff. Besides that, there's two new arenas and they've nixed the underplayed 5v5 bracket. There's also the Dalaran Underbelly which can be converted at times to be a FFA PvP zone with a few rewards involved.
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Fire mages are good right now, the other 2 specs are under-performing without artifact weapons in the pre-patch. People generally think that Fire is the most fun though at the moment in beta. Both DPS specs of DK are fairly strong, though Unholy is probably better, were I continuing to main my DK into Legion I'd probably stick with Unholy (even if I prefer the fantasy of Frost more). Simplified with the prune, but still pretty fun mechanically. Blood I haven't looked into that much, but I haven't heard too much in the way of complaints.