Let's see, a few more differences between GW1 and GW2...
In GW1 you had some quests in instanced explorable areas and then you had the larger scripted 'missions', also instanced with lobbies in which to form teams. In GW2 you have Personal Story missions (with various branches that reflect your personal choices, but the very end is the same regardless) that are instanced for you and your party but the majority of the game is played in the open world with other players. I say "open world" but really every zone is its own instance with as many players as it can hold. There are "Renown Hearts" which are public quests with personal progression. When you fill the Heart to completion you unlock a merchant who will barter your Karma (a currency gained through doing different things) for different items. There are also loads of world events that happen with pre-ordained triggers. Some of them trigger from Renown Heart completion, some trigger from interaction with players, some happen at set times of day. Some chain off of previous world events as well. But these (Personal Story missions, Renown Hearts, and World Events) are the primary source of experience gain. You also get experience from exploring, gathering, crafting, defeating enemies (NPCs or other players in WvWvW), and finishing tasks such as "Zone Completion". To complete a zone you have to find every point of interest (POI), finish every Renown Heart, view every Vista (sometimes found at the end of platforming puzzles, they give you a panoramic view of the surrounding area or a key point in the game's history), finish every skill point challenge, and unlock every waypoint. Each of these actions gives experience individually as well as a bonus upon zone completion. The bonus varies, but generally it is 2 decent pieces of gear, some karma, a nice chunk of money, and occasionally something that you would otherwise have to purchase from the cash shop such as an experience booster.
I guess that's just one difference, really, when you think about it. There's just a heck of a lot to do in the game. I leveled my second Warrior to 80 recently and have begun getting good gear for him. I initially wanted him to be just support-oriented, but then I stumbled across a build that I really like. It's still very supportive with massive buffs to group damage and some crowd control but it also deals substantial damage to single-targets through bleeds & critical hits. I'm pretty pleased with it. So now I have a Warrior that is just pure damage dealing with no real group support and a Warrior with heavy support and nice damage too. I am constantly getting praise from group mates when the enemies just melt in front of us.
Edit: Thought of another difference. In GW1 you generally clicked to move and this could be troublesome. In GW2 you use mouse/keyboard to move and there is an active "dodge" mechanic tied to endurance so that you can not just spam dodge, but you can use it to get out of sticky situations. It makes for a much more active gaming experience.
Another difference is the way skills are handled. First off, your primary 5 skills (1-5 on the skill bar) are determined by your equipped weapons. Every class has 2 weapon sets aside from Elementalists & Engineers who only get the one (one for normal combat, one for underwater combat still). You can switch between them at will or with a small cooldown during combat. Your number 6 skill is a healing skill. Most only heal the user but some have bonus effects for other players, especially with unlocked traits. You unlock the 7 slot at level 5, this is your first class skill. At level 10 (I think) you get your second class skill slot (number
and then at 20 your last one (number 9). At level 30 you can purchase an Elite skill for your number 10 slot.
The slot skills are purchased with skill points whereas weapon skills are unlocked through gaining experience from combat with the weapon equipped. There are no respecs for slot skill purchases but you will gain far more skill points than you have skills throughout the game. There are 3 tiers for the skills that reflect their cost in skill points. The first tier skills cost one skill point apiece. The second tier unlocks when you have purchased 5 skills from the first tier (generally at level 7 or
. Second-tier skills cost 3 skill points apiece. The third tier of skills opens when you have purchased 5 second-tier skills. Third-tier skills cost 6 skill points apiece. Elite Skills have 2 tiers, and to purchase second-tier Elite skills (which cost 30 skill points each) you will have to have unlocked 2 Elite Skills from the first Elite skill tier.
It sounds ridiculously complicated when written out but it's amazingly simple and the game walks you through it easily.
