Legion began with a catastrophic defeat. It was actually a really good move on Blizzard's part to sell the stakes of the expansion. There was really no benefit to our actions in that first invasion - except perhaps to demonstrate to us how ill-prepared we were.
Given the losses there - Varian, Vol'jin, and Tirion Fordring - the expansion has been building up to this new assault. No longer the two factions working only kind of together, instead we have the Armies of the Legionfall - a faction composed of all twelve class orders, who are working with far better integration, each class playing to its own strengths.
When 7.2 hits, you'll get a quest to go to Krasus' Landing in Dalaran and begin a scenario that has you assault the Shore again. You wind up taking the southwest corner of the island, and after a fight with Mephistroph (who retreats to the Cathedral of Eternal Night - the new 5-player dungeon) you claim your beachhead.
You get a number of quests sending you out into the zone - one basically has you get all the flight points while another has you travel to the lost temple area where Ret Paladins and Vengeance Demon Hunters got their artifacts (I think that's all...) This quest unlocks new levels of artifact knowledge, giving you a freebie that, assuming you're at 25 now, will boost you from 24,900% increased AP to 100,000%. And you'll be able to research more that will eventually let you hit either knowledge level 40 or 50 (can't remember what the latest build had.)
Of course, with some people at 54 traits now, you can probably imagine that they need to give you more traits to work with.
After doing a couple more quests in the Broken Shore, you'll get new quests for each specialization for which you have unlocked the first "prestige" trait in the corresponding artifact. These overlap, but no single class will have the same quest chain to do twice (there's only one tank chain, I believe, but because Priests have two healing classes, I believe there are two chains for healing.)
These quests very much feel like set-ups for later chains (perhaps these are the new artifact model quests?) as all of them seem to end with an anticlimax, but completing the chain will unlock the new artifact traits for that particular weapon (you'll need to do the quests for every artifact you intend to keep upgrading.) In addition to four new traits (one of which is a golden trait) you'll also unlock a fourth rank in all of the previously-three-rank traits. Yeah, they really want to give us reasons to keep grinding AP.
The Broken Shore zone is not small, and has plenty of world quests to keep you busy. The world quests here reward a new resource in addition to traditional world quest stuff, which is used to build the various structures on the base - this seems to be server-based, so the more people who contribute, the faster those buildings get built. The buildings have various effects that can help you out as you adventure there.