There were two feelings I really liked when trying the GW2 beta. The first is the playground between the sandbox and the theme park. The second, and I have not felt this for a long time in an MMO or even most team-based games, is that the players were all on the same team.
If the design is working as intended,* everyone on the same server is on the same team. If someone is fighting, you should help him. There is no kill-stealing. If someone is on the ground, you should rez him. You’ll get experience points and achievement progress, and then there’s someone else around to help you with the event.
In League of Legends, a team-based PvP game, there are arguments and apologies about “kill-stealing.” In LotRO, we teach friendly healers not to help people they pass unless that person seems likely to die; any assistance you give them cuts their xp in half, and you the healer gain nothing but resentment. City of Heroes has similar issues with buffs and massive AE. In systems that reward whoever does the most damage, you can imagine the anger of tanks and healers when DPS swings by to “help.” I still have golden memories of the DAoC beta discussion when healing people outside your group would net you some of the experience from a fight; Sanya Weathers dripped scorn for the concept of “grief healers” … and within the month, the game recognized it as a problem that you could run through a zone, heal people in no real danger, and leech experience from their fights.
*: Implementation is key. You can have a policy of encouraging cooperation outside grouping but also have game code that discourages it or rewards exploiting others. For all I know, I was leeching (or just destroying) experience when I helped others, but then the big rewards came from completing things, not kills. Beta weekend participants noted problems with “contribution” calculations; one player reported getting a gold medal for four simultaneous events by contributing minimally in each, while another spent more time rezzing than killing and was minimally rewarded.
Some games have the problem of having newbie A tag monsters then veteran B crush them, which yields fast levels for the newbie even at half-credit. City of Heroes used to have the following variation: veteran B would use a 0-damage AE aggro toggle, such as a Tanker aura or Radiation debuff, to gather up half of Perez Park and then jump in a dumpster; wait 30 seconds for 100 collision-free enemies to stack up and build aggro, then newbie A would use AE attacks to smite them for masses of risk-free experience. Games sometimes counter that by having low-level enemies yield 0xp when crushed by higher level foes (or without enough damage contribution), which trades power-leveling problems for a new griefing opportunity.
Implementation is key. The details of the design can easily reward aberrant behavior or punish intended behavior. Leaving the asterisk behind, I really want to be able to help people without worrying that I am hurting them.