This was a long time coming. Guild Wars 2 is—like all fantasy games are—about dragons. One of the major complaints about the game's first season of story updates was the lack of anything to do with dragons. This might seem petty, but if you've got giant dragons tearing up the place, you might think they're more important than a local election . In the end, it was all a ruse: the story ended with the awakening of a new dragon. Now the effects of that awakening are being felt. Dragon stuff is happening, and will continue to happen in the upcoming update, The Dragon's Reach: Part 1.
If you've not been following this second season, you might be confused at the lack of dragon in that trailer. To which someone following the game's lore would point out that the tentacles are the dragon. A bit of it at least... the tentacle bit.
There's little information about what the update will introduce, although the game's Living World structure has been very different this year. Most of the non-story events have been centred around the new zone of Dry Top, but in addition, each release has included a multiple-part episode purely focused on new story. It's been a nice switch for the game: where many of the first season's releases were pretty farming heavy, this time, there's a clear path through the majority of an update's content.
Essentially, then, we can expect another few hours of extra story, and potentially some new areas and events. The trailer certainly hints at escalation: from Rytlock's return, to Ascalon shenanigans, it suggests the story is all leading to something. There's even what looks like an Ice/Plant monster hybrid thing, which hints at... something.
The Dragon's Reach: Part 1 will be the penultimate fortnightly Season 2 release before the Living World goes on a mid-season break.