The main issue with Diadem boils down to two big problems and everything spins out from there.
1. This content was never designed with duty/party finder in mind. It was soley designed for Free Company groups with common insterests to go in, gatherers would be protected by their charges to assure quick and bountifull harvests and when the time came to gather for a fight it would be a no brainer.
2. The shared claim system: Only encourages people to zero in a single area and just spam-fest for 90 minutes there is nothing to encourage people to split up no reward for doing so. There needs to be a party/alliance based claim system and give rewards to ALL players in a zone if spreading out happens, more Star Ranks Spawn faster to allow battle alliances plenty to battle while giving more of a challenge with a single group per monster and more loot oportunities for all.
There are other minor issues such as making Gathering more team play rather than just greed, part of that could be making them important part of spawning some Star rank mobs, and other things such as perhaps buffs for the team with certain rare harvests. There is alot of work that could have been done but wasn't.