As others have said, a healthy guild is a good solution which permits a lot more freedom in how you play and what you play, and ultimately makes all experiences in the game more fun.
However, roles don’t need to be defined by stripping them down and forcing them to rely on other players to be successful, they need to be defined by promoting particular strengths within certain areas. Player A doing multiple roles simultaneously is not a problem if player B can excel at one particular role better than they could. And when I refer to roles I don’t mean tanking/healing/dps, I refer to control builds, condition stacking, support, high damage power builds etc. In this regard, I think the games balance is more or less fine with the exception of ranger that just doesn’t feel strong enough in any particular area, but still does okay for its versatility. [should note that the reason power/zerker builds are popular lies in the balance of damage types, not balance of classes/roles]
The example of ranger is important though, as it extends to all classes in all areas of the game. You can literally play anyway you want because of how each class is independently responsible for its own power and role within a team, which means that build diversity and player role is vast. If you want to roam on a necro you can, and even though you detailed that Thief/Mesmer ‘excel’ in these roles, they still rely on player skill, build, equipment etc. to be successful. On the flip side, a Necro/Thief/Mesmer can also build themselves for group oriented combat quite easily, and successfully. Group interaction and the satisfaction of working with others hinges entirely on the group of ‘people’, not roles. If you are joining a dungeon speed run and you don’t satisfy the group criteria of killing things fast then you are in opposition of the other players yourself.
Again, in answer to your point on Guild Events it depends entirely on what guild you run with. If you’re in a guild with a WvW focused commander then you will have a lot of opportunities to play with people you like on a daily basis. I don’t see opportunities to play exclusively with your guild as a reasonable excuse for the lack of cooperation in the game, though I will concede that daily guild events etc. would be awesome. Also, if you haven’t done any guild WvW runs I would thoroughly recommend it; you can roam as a small group or zerg if you have more players, and using voice comms its a very enjoyable experience.
Your comments on melee are true to an extent; the range thing I never really realised, but its true. I suppose that as a compromise to having dynamic combat there really needed to be a way for melee to be competitive; in WoW for example, there are many problems with the balance between melee/ranged, with ranged generally having an advantage in fights because melee is unable to close them down enough. I think the balance is fine in GW2 though, but I play WvW mainly and movement/positioning is critical during a fight. “Then you are told you are playing wrong” is a troubling thing to read as it implies you’re playing with the wrong people; role diversity is a staple of GW2 combat and is part of its charm, so you are in essence never playing the game wrong. Min/maxing is player choice, and it shouldn’t impede on a players ability to play with others and have fun.
My post is written from someone who focuses on WvW more than any other aspect of the game, and compared to PvE there is far more incentive to group up, communicate and work together to achieve success, but that’s not to say its impossible to have that in PvE, it just requires more effort on the players part, as it is in any other game. I’ve played WoW on and off since Vanilla and participated in raids, dungeons, arenas, battlegrounds; you name it, I’ve done it, and the most satisfying part of the game to me are the opportunities to play solo. Cooperative gameplay, teamwork, group activities; it doesn’t mean anything if it isn’t fun, and the dynamic fighting and being able to change roles at any time is what makes GW2 enjoyable to me.