Oh I finished this on Normal on Release, and completed Legendary a few days ago solo...
Initially, I was a bit "WTF" on the whole thing. The end was very out "there" for my liking, with some pretty big leaps of faith 343i are hoping the audience will take. Plus it felt even shorter than Halo Reach for my liking. BUT, since playing through again on Legendary...the story has grown on me. I'm still a bit wtf on the ending, but I don't let it overshadow the rest of the game.
What about the ending was a WTF moment for you?
If you have any degree of familiarity with the extended Halo fiction, you know that Cortana
so that shouldn't have come as a huge surprise.