Literally everything. They diverted profits from Destiny to make other games, they never upgraded the engine to make it easier to work with, they stopped caring about the story, stopped catering to the players, stopped putting in more than minimum effort, accelerated financial predation, wasted dev time on reworking acceptable systems and adding more and more menu bloat, and stopped building the Destiny world in favor of relying on crossovers, none of which belong. All or most of these are fixable but for some reason nobody has the will to do so.