What was Big Willy Shakes going for?
Iago is all about not revealing his true identity or intentions to anyone. He plays his cards as close to the vest as possible. Here, he tells Roderigo that he'll never allow his "outward action[s]" to show what's really going on inside of him because that would leave him vulnerable, kind of like allowing a bird ("daws") to peck at his heart.
When Iago says "I am not what I am," he cryptically suggests that he's not what he appears to be. This phrase, we should point out, is an inversion of God's line, "I am what I am" (Exodus 3.14). Hmm. Is it just us, or does it seem like Iago is aligned with the devil?
When he says he'll wear his heart on his sleeve, he means he will show his real emotions and feelings. Irony alert! Iago is never what he seems, and he certainly never shares his emotions. We're thinking it's opposite day in Shakespeare land.
Even in Iago's more revealing moments with Roderigo, Iago is still playing him, and sometimes, us as well. He knows exactly how to manipulate people, so when he says he will soon get to show his real feelings to people, we're not quite sure we believe him. (And you shouldn't either.)