I've gained some new found appreciation for that Eren and Armin dialogue as they walked through that dark forest along with the other Survey Corps members. It was a nice character building moment between the two characters as Eren expressed his doubts and fears on what was coming, with Armin giving him the pep talk he needed along with some nice affirmation for the freedom from the walls the two characters yearn for.
Armin too was just a star throughout the episode. His leadership and tactics as he told the other Survey Corps members to start scouting the enemies within the walls shows how far he has developed as a character, with even Erwin recognizing what a valuable asset he has become for the military. Armin initially being intimidated as he tried to take on a leadership role is something I can relate to a lot with my current work as well, at least in situations where your boss suddenly hands you the leadership for something you did not expect to take charge of.
Also, it sounds like he picked up the scent of coffee when he found those three mugs and that pot along with the "black liquid" found inside it. The brief dialogue about the pot not being to handle the tea kinda makes you think: coffee doesn't exist within the walls, but what they describe is something the viewers can recognize.
Then we learn Reiner was hiding behind the wall, and damn did I get chills when he made his ambush! The anime did a great job adapting that scene from the manga! I also liked the nice little added detail of Reiner's pupils coming back as Levi tried to slice his nape, hinting at the Titan power Reiner used that Levi noticed.
It was an awesome first episode overall with some great character moments, nice pacing, great animation (the CG used with the walls mixed well with the Survey Corps arrival at Wall Maria), and just a perfect adaptation of the particular manga chapters covered to the point not one scene or line was missed.
As for the opening and ending:
Damn, that opening is kinda spoiler-y compared to the other openings! There were some good shots too, like my girl Historia toward the end! Music is solid as expected of Linked Horizon. I don't enjoy it as much as Guren no Yumiya as much as their first three openings, but I think I'll warm up to it the more I listen to it. Probably a second or third listen will make me appreciate it as much as the others.
Ending was pleasant. Didn't expect that it would involve a montage of training corps memories, but I can see what they were going for here considering this arc has this climax feel, and thus it could just be a "look back" to what our heroes have gone through before the climatic battle takes place (kinda like the opening to the first half of Season 3).