The people of Judah have forgotten who they belong to, and therefore, have forgotten who they are. The result of this identity confusion presents itself through corruption, guilt and rebellion. This is true of society today. Those who do not know who they are suffer as well. They chase the offers that the world deceivingly provides, which leads to a life of sin and emptiness.
What is interesting is that verses 15-17 illustrate some detail of this rebellion through illuminating the issues of burnt and sacrifical offerings. The Lord is not happy with their hypocrisy.
Verses 18-20 are my favorite because they bring back hope into the equation. Ultimately, the verses portray God's character as the perfect father, and I love this. What's startling is that Judah is blind to what God is offering them. He's giving them the chance to repent, and in doing so, is promising them the best food in the land (and probably more than just food). How could they not want this!
For the first time, I see an aspect of God's loving fatherly heart in this chapter in Isaiah. He is simply being a wonderful father by offering them good and redemption, and yet at the same time, he warns them of their consequences if they continue in their rebellion. But I don't think this is "big" "mean" God speaking to Judah in Chapter one, I think it's "loving" "perfect" Father talking.