The quests unique to the Gathering Hub are Arena Quests. You choose from four predetermined loadouts of gear and items, each specific for that quest, and you get graded for your time. Your rewards are mostly Arena Coins necessary for the Weapon Workshop tree of weapons and the Brigade armour set.

I think the monsters are scaled for multiplayer even if you're solo? I've only done a couple. I wouldn't worry.

While Kulve Taroth was around her Siege was only available from the Hub, but from the standard Hub Quest Lady. The Arena has its own Arena Lass.