Most of the information provided on EverQuest Next falls into the "things we already knew" category: The world is procedurally generated, destructible, tiered, and made of voxels. AI responds to your actions in lieu of sticking to specific spawn points; for example, Orcs will naturally search for areas with low guard patrols and high traffic so they can make the most of their thievery and will leave for greener pastures should you start making their work difficult by killing them or alerting guards.
There were new details on Landmark, however (check out this great post from MJ to see what we knew before). You can choose between male and female heroes and will begin as an Adventurer class at one of the world's great landmarks. From there, you can adventure freely into the world, find a spot you like, and claim it as your own. Once your spot is claimed, you can build on it however you see fit or move on to claim other areas and link them all together.
Landmark will feature many common MMO functions. You'll be able to start guilds and make friends, along with creating temporary build projects with others. You can also gather resources, craft items (including better gathering tools), and man an in-game booth to sell stuff to other Landmark adventurers. You can even sell your real estate on the global marketplace or make a career out of crafting environmental items for other players to use.
One area of Landmark will be reserved for art in the Norrathian style. Players can use the game's tagging system to "like" content, and the most-liked content (that fits the art style) will show up in EverQuest Next proper on day one of its launch. If you play Landmark, you'll also have access to the Adventurer class when you boot up EQ Next, allowing you to start multi-classing from your first moments in-game (or you can just transfer your Adventurer directly into Next).
You can learn about these features and more by checking out the video-on-demand of SOE's presentation. The Landmark stuff begins at around 13 minutes.
[Thanks to Paul for the tip!]