I was expecting the fall of Reach to be the whole season. I assumed the second half was going to be Reach under siege, culminating in the Covenant glassing the planet and Chief escaping on the Pillar of Autumn with Cortana.
But most of the changes I didn't mind. As you said, this isn't the game and I'm happy for them to diverge from the finer details of the established storyline a bit.
That said, I'm not sure how I feel about them bringing the Flood into the show as early as this. I really do wish they had stuck to the game for that and had them show up half-way through the next season.
But they look great and I'm even more interested to see hordes of them. The bit where you saw one of the small parasites shoot up into someones eye socket.
I still didn't care in the slightest for Quan Ha and her storyline. When she showed up at the end of episode 1 it was definitely a case of "Oh yeah, she exists".
Same for Soren and his family. That whole second half plot where they were looking for his kid should have been cut entirely and replaced with something more relevant. Again, I would have liked to have seen more of the Fall of Reach, but I guess the budget can only do so much.
I loved the conversation at the end, but I really wasn't sure who it was going to be. Logically I figured it was going to be 343 Guilty Spark, but the dialogue reminded me more of Gravemind.
Either way I'm glad we're finally on Halo. It'll be interesting to see where it goes from there.