1.) What exactly do players do at level 280? (I have inspected many players and I rarely ever see there gear +1 +2 +5 etc… Why don’t they upgrade?)
Earth Tower/World Bossing and right now we have PvP to have fun. I think we can’t see people’s upgrade gears.
I think it’s also too early to +1/+2 gears because it costs a lot. The better your equipment is the more you’ll spend to get it stronger. So once there are a lot of new equipments and sets to come I suppose it’s not the best choice.
2.) I keep hearing that past a certain level, the game is 100% group based because NO ONE can solo. Is this true? Does it get to a point where any class I pick must be grouped to kill stuff?
Well it’s possible to level up while soloing but it’s not a good idea because grouping you take EXP Bonus from classes. That means you level up faster and less stressful.
3.) If #2 is true, shouldn’t we all design our characters for either group play or PvP or both?
Late game people prefer to have AoE classes in party to make things easier and it’s not always possible to happen. Anyways you can design any class or whatever you want and some classes will be easier and faster to level up and some might be tough.
4.) I keep killing bosses during story quests that sometimes drop cards… Is there another way to get there card if I don’t get it from questing? How important is the card system?
Card system from now on it’s possible to summon that boss if being Sorcerer and Wugushi also gets some things from Manticen. I don’t know exactly how it works. Very soon we’ll be able to attach our bosses’ cards to have new attributes! That sounds cool. 
5.) I see a lot of players build for AoE. I know AoE is king for leveling up, but what about bosses in this game? Are they very hard? Are they hard enough to want a boss focused single target attacker in your group? Are bosses soloable?
Some bosses are soloable because they have only physical attacks and don’t do anything to get you killed. Maybe it has a huge amount of HP but nothing else. High Level World Bosses are the tough ones and some of them have strong magic skills, so it’s quite hard and you’ll probably need a lot of people to kill them. You can search for Rexipher, Marnox, etc.