Is the initial "who am I" reaction in these narratives supposed to be taken positively as a sign of humility in the character? Or negatively as a sign of a lack of faith? Neither?
The best way to know if the response is positive, negative, or neutral is to look at the context and see the response of God. Humility is from the heart and only God knows our heart.
Digging into the context:
Exodus 3 (NIV)
10 So now, go. I am sending you to Pharaoh to bring my people the Israelites out of Egypt.”
11 But Moses said to God, “Who am I that I should go to Pharaoh and bring the Israelites out of Egypt?”
12 And God said, “I will be with you. And this will be the sign to you that it is I who have sent you: When you have brought the people out of Egypt, you will worship God on this mountain.”
13 Moses said to God, “Suppose I go to the Israelites and say to them, ‘The God of your fathers has sent me to you,’ and they ask me, ‘What is his name?’ Then what shall I tell them?”
14 God said to Moses, “I am who I am. This is what you are to say to the Israelites: ‘I am has sent me to you.’”
Later in Exodus 4
10 Moses said to the Lord, “Pardon your servant, Lord. I have never been eloquent, neither in the past nor since you have spoken to your servant. I am slow of speech and tongue.”
11 The Lord said to him, “Who gave human beings their mouths? Who makes them deaf or mute? Who gives them sight or makes them blind? Is it not I, the Lord? 12 Now go; I will help you speak and will teach you what to say.”
13 But Moses said, “Pardon your servant, Lord. Please send someone else.”
14 Then the Lord’s anger burned against Moses and he said, “What about your brother, Aaron the Levite? I know he can speak well. He is already on his way to meet you, and he will be glad to see you.
Moses flat out did not want to do what God wanted him to do and it was in no way positive.
In Judges 6 Gideon has an on going conversation with God:
17 Gideon replied, “If now I have found favor in your eyes, give me a sign that it is really you talking to me. 18 Please do not go away until I come back and bring my offering and set it before you.”
And the Lord said, “I will wait until you return.”
36 Gideon said to God, “If you will save Israel by my hand as you have promised— 37 look, I will place a wool fleece on the threshing floor. If there is dew only on the fleece and all the ground is dry, then I will know that you will save Israel by my hand, as you said.” 38 And that is what happened. Gideon rose early the next day; he squeezed the fleece and wrung out the dew—a bowlful of water.
39 Then Gideon said to God, “Do not be angry with me. Let me make just one more request. Allow me one more test with the fleece, but this time make the fleece dry and let the ground be covered with dew.” 40 That night God did so. Only the fleece was dry; all the ground was covered with dew.
At no point do we see God's anger toward Gideon. God's response demonstrates God's knowledge of Moses or Gideon's heart. Moses warranted God's anger and Gideon was granted sign after sign to confirm his task and encourage him.
We don't see God's anger toward Gideon, but we don't see God's pleasure either. God's response to Gideon is neither positive or negative but more patient, gracious, and affirming.