I think part of the concern is that it isn't clear cut how much dps is acceptable. I'm sure everyone would agree that something like 400 is terrible for a 60 dps, but what about 800? 1000? The answer will likely change depending on who you're talking to, the content itself, and other factors such as how geared the dps is.

With tanking and healing you can say that a player has done their job if they're holding aggro and keeping their group up and doing a great job if they're adding dps on top of their responsibilities.
Most content in the game doesn't have tight dps checks so it isn't as easy to judge objectively. In my own experience I've only had one group with dps low enough to fail a dungeon mechanic (the jails in the Aery) and we got through it after I started helping on my tank.