8.1 introduces Faction Assaults. Like Legion Invasions, one zone comes under attack for a set period of time. New world quests appear, and after you complete four, you do a final quest to finish the assault.
There are some changes from Legion. The Assault World Quests don't replace existing World Quests. They are all clustered in a specific part of the zone. There are different (but similar) quests for each faction.
The assaults are well done. The zone changes, and it really feels like a major battle is taking place. For example, in Drustvar, the city has magic shields fending off Horde bombardments.
These Assaults are excellent in War Mode. I turned on War Mode to do the kill 25 Horde quests, and it was a lot of fun. There are Alliance groups running around clashing with Horde groups. The Assault structure funnels everyone into the same locations, encouraging lots of faction combat while doing the World Quests.
I even joined an Alliance raid group in Zuldazar that got hyped up and decided to sack the city of Dazalor. Not really sure why, but we invaded the bottom of the city, killed everyone at the inn, then charged all the way to the Great Seal and killed everyone there. It was pretty hilarious.
Even though I had a fair amount of fun in raids, I still don't think they're a good match for War Modes. It's just too uneven. I was in a raid of 25, and we encountered a raid of 10, and wiped them out. I strongly believe that War Mode would work better if they were somehow restricted to 5-man groups. Even multiple 5-man groups loosely coordinating would be better. It also might improve performance a bit, having several 5v5 fights scattered around, rather than a single 40v40.
I'm not sure how Blizzard would do that though. Perhaps War Mode is automatically disabled while in a raid. I have a feeling that there would be a huge outcry though.
In any case, Faction Assaults are quite fun. I strongly recommend turning on War Mode for them, and joining a small group.