Just finished it a second ago. Overall I think it was a really great game, just a bit more explanation of stuff story wise would have been helpful.
Was it ever explained why the Covenant were even at Requiem and going all 'gung ho' on a badly damaged UNSC ship? Are the Humans and Covenant at war again? Was it just a rouge group? I think I remember a brief statement where either Chief or Cortana asks why they're attacking because they thought they had a truce and the other replys that 'a lot can happen in 4 years' and then the Didact brainwashes them or something so then they all fight for him and it's never mentioned again.
The Librarian cutscene was a little wtf but like the OP said it gets a bit clearer when you see the composer in action, which brings me to my next point. The composer was being fired at earth for quite a while surely there would of been at least a few casualties? But its never mentioned. And if the composer is meant to turn humans in prometheans why did it face melt those scientists on that facility near the other halo ring?
Im sure there are some other things I was unsure of but that'll do for now.