Welcome back! There have indeed been a lot of changes to the game with HoT. I’ll preface my response by saying that I’m almost solely a PvE player so I can’t offer any help with either PvP or WvW.
The mastery system is one of the biggest changes you’ll notice, and it can be confusing to figure out how it works. In short, it is an account-wide progression system that gives your characters new ways to interact with and move through the world. When you enter the expansion maps on a level 80 for the first time, it will unlock the mastery tab in your hero panel so that you can select the mastery that you want to work on. To earn masteries, you will need both xp and mastery points. Any xp you gain on a level 80 will go towards your selected mastery. Once your xp bar is full for your selected mastery, you can then spend mastery points on it to gain that mastery. Mastery points are earned through achievements (look for the achievements in your hero panel that have the red or green starburst thing next to them) or in the open world at locations marked on the map with a green starburst. Keep an eye on your xp bar; as soon as it’s full, you’ll want to spend your points then go back and select the next mastery you want to work on.
Note that there are two mastery regions, Heart of Maguuma and Core Tyria. Any xp earned in the expansion maps will only go towards your selected Heart of Maguuma mastery track, and your xp earned in the core game maps will only go towards your Core Tyria mastery track. Likewise, the mastery points themselves are separate; you cannot spend Core Tyria mastery points (the ones marked with a red starburst in your achievements panel; the ones marked with the green starburst are HoT mastery points) on HoT masteries, or vice versa.
As a side note, when unlocking HoT masteries, I generally recommend this order to facilitate exploration of the new maps: glider basics > bouncing mushrooms > updraft use > exalted markings > nuhoch wallows. It doesn’t require much in the way of mastery points (though it may take you a while to earn the xp for them) but makes your life much easier when you’re trying to get around the jungle and will allow you to explore the vast majority of the new maps.
As a core game veteran, the other thing you’ll notice is that the difficulty level in the new maps is greater than in the old maps. The mobs in HoT hit harder than you’re probably used to from the core game, so you may need to adjust your playstyle accordingly. More use of active defenses (and you have plenty of them as a mesmer!), dodging, and movement will go a long way to helping you survive the jungle. Be prepared for a lot of dying as you get used to the new mobs and their attacks, but don’t let that prevent you from learning their tells and how to fight them!
Another big change that I want to mention is break bars. It’s a new mechanic introduced with HoT, and in my opinion is poorly explained in game. Remember the old defiant stacks that bosses used to have? Those don’t exist in the new maps. Instead, enemies now have break bars that appear as a secondary bar underneath their hp bar. The most important thing to know is that you want to save your CCs (crowd control skills — daze, knockback, stun, etc) for when that blue bar appears. When the blue bar appears under the enemy’s hp bar, using CC will lower that bar. Throw enough CC at them and you’ll “break” the bar, which results in different things for different enemies, but mostly involve ending or preventing a big attack, or them taking increased damage.
Finally, as a mesmer, you’ll want to fully unlock and equip chronomancer as soon as you can. Chrono gives you a baseline 25% movement speed boost, gives you decent aoe with access to wells, provides alacrity to reduce your cooldowns, and can up your damage through shattering and respawning phantasms. For instanced content, our (I main a chrono) DPS is still at the bottom of the pile, but chrono is in demand for pumping out alacrity and quickness to party members.
Good luck, and have fun!
