Guild Wars 2: Heart Of Thorns confirmed info by MedicantBias66

TL;DR
New profession, new group challenges, there will be 3 parts of the Maguuma jungle to explore, the treetops, the heart, and the roots. No level cap increase. A new mastery system, profession specialization, and other cool stuff.

Pricing/Release Info: Expect this to change (Seriously, expect it. @.@) Pricing is currently not available, but it will be a paid expansion, not a free update
Release is also currently not available. Read on for more info :D

 
Mastery System: The new account based PvE progression system available to every character level 80, challenges completed in the jungle gives mastery points such as challenges and group challenges, as well as map completion.
Achievements also award points and Mastery points can be spent on abilities for all characters on your account and don't need to be repurchased.

 
Exploration: Jumping from the highest point in the map with an ability purchased with mastery points will allow you to hang-glide instead of going SPLAT. Helping with exploration as you purchase more hang-gliding abilities to fly over wind currents and take you to new vistas and explorable areas.

 
Lore: Hidden access for lore related stuff, such as only accessible areas for mastering a language.
The Mursaat: Our allies or our new foes? Either way, the floating false gods of Guild Wars: Prophecies are now (possibly) making their way back into the Guild Wars world in Heart Of Thorns.
source

 
Combat: Building on previous combat system, the mastery system will allow you to progress your character as you fight harder enemies and bosses throughout the jungle.

 
Crafting And Building: Mastering Crafting Masteries will allow you to build collections that once completed, reward you with precursors, getting you "started on your journey to your legendary" along with new precursors and legendary weapons.

 
Specializations: Profession specializations will function as a sort of secondary profession, for example, rangers mastering druid abilities, to become a druid.
The ability to use a staff weapon and have access to a new elite, ultility and profession mechanics that change the way the Ranger plays if they become a Druid.
Continuing that, one of the Necromancer's specializations will be able to make use of Greatswords. Between now and the release of the expansion, all profession specializations will be revealed. "When its ready"(tm).

 
NEW HEAVY ARMOR PROFESSION HYPE: The Revenant The revenant profession is the third heavy armor profession. The main example being Rytlock as he chased after his sword in Living Story Seasons 2. The Revenant will have the ability to control and channel powers of the mist.
In combat. the powers of the mist will be used to channel in the Revenant's weapons using the "power of great legends past". Revenants will be able to channel and change their skills by channeling. Legends are the term used for the "power of great legends"

 
PvP/WvW: Brand new WvW borderland map, to be added to the rotation. The new borderland is the "most diverse" map, as each keep has a theme and holding objectives around the keep provide abilities to those holding it.
Such keeps like the Earth keep will raise up parts of the ground to defend itself and the fire keep will provide magma pools for your group to use to travel around.
Changes to core WvW involve changing how holding objectives affects a sense of "victory" and importance to holding the objective.

 
GUILD VS GUILD CONFIRMED Competitive PvP, Small teams: Brand new gamemode called Stronghold, this gamemode is reflective of GW1's GvG game mode, you will be attacking a guild lord with forces and heroes you recruit on the way to attack your enemy guild.
When battling in the new gamemode, you will be able to form Guild Teams, registering on a Guild Leaderboard, to "prove who are the best guilds in the world."

 
Guild Halls: While last on my list but certainly not least (as I was typing almost word for what what was said), Guild halls are making a return to the Guild Wars world. A guild hall is "much more than what it is in the past"

A guild hall is much more than a place to gather, it is a place to meet and party and a place to progress as a character and a guild. Preparing to face new challenges and provide you with a home base for "years to come".