You can pick apart the problem in a couple of different ways; from the team perspective and from the individual perspective.
From the team perspective, we are a poor rebbounding team, mediocore defensively, and poor to mediocore shooting team. These all lead to a poor to mediocore team.
From the individual standpoint, we do have a talent issue. At this point, we have no stars. Rondo is very good, but not a star frankly. KG is closest, but can't play big minutes. And PP, simply is old.
Role players (which is everyone else) can only shine when they have star players to play off of. When they don't, they are exposed, ineffective, or inconsequential.