My main is a tankadin and my Offspec is DPS. So I'm not throwing Ret's under the bus. I actually ran an instance today (ZA) and went as Ret because I didn't feel like dealing with the stress of tanking.
Saying ret is a weak spec is a generalization, but it's a fair one. On any given Sunday, you can find the best raider as any class, but generally speaking Ret is at or near the bottom.
From what I've seen from my own personal use of the spec - not only is it weak DPS wise, but the entire design seems broken.
I really wish it was different. I enjoy DPS. Also - my guild uses me as the tank most of the time, but I switch to DPS on fights like Valarion (twin dragons). I have never been a good healer, but the point is kind of moot with my guild. They don't want to drop a tank, and sub in another healer.