Generally I look at guys relative to their position when determining how good they were. The greatest hitting catcher of all time should easily be a HOFer. To do what he did offensively, while playing catcher, is ridiculous. There is a reason why you don't see catchers put up offensive numbers like 1st basemen do.
Also, aside from his throwing arm, I'd say Piazza was good to above-average behind the plate. Pitchers loved to throw to him. Coming from someone who watched a shitton of his games.
And to the guy who asked why I thought it was interesting that Piazza wasn't getting votes that Thomas did, it's because Piazza isn't getting votes because he looks like a roider. I don't understand the difference between what Piazza looks like and what Thomas looks like.