Preface
I've not played TERA, so I can't do a comparison. I've also only played BWE1 of GW2 recently and I tried Guardian and Elementalist.
Aim and Dodge
Guild Wars 2 has been specifically designed with active combat in mind. As such you must keep moving at all times when in combat, either by circling your enemy, backing up, or using Endurance to dodge.
If you stand toe-to-toe with enemies you'll get put down quickly.
If you can circle enemies and stay mobile it's possible to put down +3's without even taking a hit! I played a Guardian in the BWE1 and using a Scepter & Shield combo it was possible to take on large groups without taking much damage just by making sure to run around lots.
The main thing to remember is the 5 D's of Guild Wars 2, dodge, duck, dip, dive and dodge! Just remember that your Endurance is limited and you'll only be able to dodge about 3 times in a row before it'll run out. I would strongly suggest strafing around enemies, even in melee and only using dodge to avoid large attacks.
As the developers have said, the combat is designed to be very active and you'll need to watch your enemies animations to spot when attacks are coming in order to evade them. The best example is Ettins using their big smash attack, where the animation starts with a gathering flash around them before they launch the attack. It's visual cues like this which are your keys to using dodge effectively. Check out the Ranger's Serpent Strike skill video.
Also be sure to understand the ground markers. White are positive effects and will usually grant a boon (positive buffs) to you. Red markers are enemy AoE markers and will usually damage you of give you conditions (negative debuff), so you will need to run out of these areas or use dodge to roll out of them.
Targeting and Hitting
As for aiming, your attacks are always targeted at your currently selected target. So as long as you have an active target you will not need to target your attacks. There are a few types of attacks which do need targeting, usually Targeted AoE attacks. When these skills are activated they will change your cursor into an image of the area, which will allow you to place the skills effect. Do be aware that there is a fast-cast option in the game options which will activate these skills at your current cursor position, thus avoiding the need to hit the skill and then place the effect. These types of skills also apply to walls, which in some cases are automatic, like the Guardian's Line of Warding and some are placed like the Elementalist's Flamewall.
Do be aware that if you do not have a selected target you will still be able to activate skills and they can damage enemies but you'll struggle to deal proper damage and in most cases will die. This happened to me a few times in the BWE1.
Ranged and Melee
This is a hotly contented issue and spawned a huge thread on the forums. Mainly as most people felt that Ranged was far more useful than Melee in the BWE1. The developers agreed with this and said that the dynamics of ranged vs melee will be tweaked as the game progresses.
The function of the two modes of damage dealing are the same. They both have a range, which you can see by looking at the skill bar, if the number is white, you can use the skill, and if it's red you are out of range.
There are a few points to note with some things such as scepters shoot a projectile which tracks the ground, thus you are unable to fire it up or down. Rangers have a huge range and pretty much everyone just stuck with Long Bow in BWE1. I ended up using Scepter on my beta Guardian simply because it was easier than using a melee weapon. I tried Greatsword, Mace, Hammer and Sword, but none of these were as easy. Don't get me wrong, they were all effective, but you'll take more hits from melee enemies as you can strafe/kite them around.
Thanks
If you read all this and got here, reward yourself with a brew and a bikkit.
Best Answer
If you have already linked your Guild Wars 1 and Guild Wars 2 accounts, them when you log into your GW2 character, you should have an item called the Hall of Monuments Portal Stone, which looks like this:
Follow these steps as outlined in the Hall of Monuments Support article:
If you do not see the HoM Portal Stone in your inventory, you can follow these instructions:
The stone is account bound, not character bound, so you can put it in your bank and use it on other characters.
As of the headstart, there are currently known issues with the HoM Portal Stone not showing up for many people and also with people having trouble receiving items from Kimmes the Historian. Reportedly, some people have fixed this by logging into their Guild Wars 1 account and then logging back into Guild Wars 2.