I got a new insight from the thread on DADGAD vs Vestapol. Namely that open tuning pulls you into a mode if not an actual key whereas DADGAD and standard tuning don't have a specific mode attached to them. I think, besides different sonorities, that is a reason the same thing arranged in open tuning sounds different from it played in standard. It also justifies the claim of some that DADGAD is an alternate standard tuning.

Murray