I understood that part, but I had absolutely no understanding of his motivations, his background, or his plan. From my perspective, he was just some monster thing that hated humans and attempted to wipe us out with some magical superweapon.I did think that the Librarian kind of came out of nowhere, but I didn't have any issues with the Didact since it seemed pretty clear he'd been trapped inside Requiem all that time.
From playing the earlier Halo games, I had no idea that humanity had an interstellar empire 100,000 years ago that rivalled the Forerunners. That's a very important fact to leave out of the games, don't you think? But Halo 4 just expects you know this fact, and you're expected to know about the Didact and the Librarian. Well I didn't know all that, and when I read up about it after finishing the game I thought the whole story was an outlandish retcon.
It leaves me with the feeling that 343i have a different vision for Halo than Bungie had and are willing to distort the lore to fit the story they want to tell.