There’s a huge background section which focuses on the Ultramarines. The Ultramarines are really being pushed as THE ultimate Space Marine chapter, which, in a way, they are.
This is bound to anger some of the other founding chapter players, like Blood Angels, Dark Angels, Space Wolves, etc. But there’s a reason why the Ultramarines are dominant. They came out of the Horus Heresy largely intact, which cannot be said for the other loyalist chapters. Their dominance shows in the fact that 3/5 of all Space Marine chapters are descended from the Ultramarines, and that the Codex Astartes is so prevalent. Some players may argue that the Ultramarines are "boring." Boring they might be to some players, but they make up for that with sheer power. If you want to play one of the more "characterful" chapters like Blood Angels, Dark Angels, or Space Wolves, that’s perfectly fine, but don’t complain when the Ultramarines are described as being more powerful. Those chapters are cool because of their character, not their raw power, and that’s why you wanted to play them in the first place. Still, a line like this is bound to cause some legitimate gnashing of teeth: "Some [chapters], such as the Blood Angels and their successors, strive to be worthy of Guilliman’s legacy, but their recalcitrant gene-seed drives them ever further from it." (p.24)(emphasis added) I would think that the sons of Sanguinius would want to tell the person making that statement that Guilliman’s successors should be striving to be worthy of the legacy of their angel-winged primarch, rather than the other way around. But then, Codex Space Marines is a book about the Ultramarines and their successors, so some creative license is expected.
Some of the background seems to have changed for some chapters, directly contradicting previously published works, for example:
Here’s a list of the things that jumped out at me while looking at how my armies would be affected by the new codex: