First, I loved the revised opening theme. The old-school Lucasfilm logo. The blood-red title card. The Star Wars theme by John Williams instead of the CW theme by Kevin Kiner. The lack of the "fortune cookies" (which, to me, always felt trite). We haven't even gotten to the story yet, and already I've got chills!
In the first battle, Anakin's seemingly reckless confidence was fantastic. This is finally somebody that I could see being charismatic enough to command the attention of his peers, yet arrogant enough to show the hints of his eventual fall. Also: "You did great, too, Obi-Wan. Your cowering in fear totally sold them on my fake surrender." HA! Hilarious!
Although I never really connected with Anakin and Ahsoka until now, I believed in their friendship here, and the brokenness of it was deeply affecting. The stuff with the clones and Captain Rex honoring Ahsoka upon her return was great, too.
To be honest, I was expecting a lot of scenes of Revenge of the Sith played out again from a slightly different angle/perspective. To my surprise, they didn't go there, but in retrospect, it made sense that they didn't waste time giving us the same thing we've already seen. The tight focus on Ahsoka and Rex drove the narrative very well. And there were those brief moments where the story did intersect with the main plot, just to keep us on track.
I'm glad we finally got a direct explanation for what Order 66 was. Not that it was too hard to guess, but such an important turning point seemed rather glossed over in RotS, so I was grateful for a bit more continuity here.
It did seem like a bit of a missed opportunity not to make the main brainwashed clone trooper someone that we had a deep connection to from earlier episodes. It would have made it much more devastating to see someone like that turn on the characters we like, and would have aided in our sympathy when Ashoka says she doesn't want to kill them. But people who have seen the whole series said they barely remember Jessie from other episodes. But that's just a quibble.
Maul has never been better. Thoroughly intimidating, yet given a depth I've never seen before, and actually put in a scenario where we can oddly sympathize with him! Ahsoka shouldn't join him, of course, but he was telling the truth about Darth Sidious, and he did serve as an effective distraction for Ahsoka's escape. The scene where he's taking out the clone troopers in the hallway was amazing. Why is Maul so much better at this than the Jedi masters were?
Also, Ahsoka's and Maul's duel was motion-captured, and Lucasfilm got Ray Park back to do Maul's movements!
The final scene with Vader was beautiful. No dialogue was used or needed; the visuals and the music were enough to carry us along.
Anyone else think that Vader was on Hoth? It sure looked like that, but it wasn't identified in the episode, and the wikis just refer to it as "a moon" instead of a planet.
Instead of starting with a theatrical movie back in 2008, maybe Dave Filoni and co. should have built to these last four episodes and then released these as a movie. Of course, it's probably just as well since going to the movies wasn't an option by the time the episodes were released, but in theory, it could have been very impactful.
Another random thought: Since the Genndy Tartakovsky series built right up to the opening shot of RotS, I was afraid that the new show would contradict it. But it didn't! In some way, the GT series could probably fit.
For me, the most essential storylines are probably:
The New Padawan
Ambush (just a great, fun episode)
The Hidden Enemy (I should probably work in some Rex stories here, but I don't know which ones yet. I'll have to use Canon Editor's guide above to try to find the right ones.)
The Mandalore arc w/Satine from Season 2
The introduction of Savage Oppress from Season 3 (I haven't seen this yet, but just so we know who he is when he rescues his brother)
The return of Darth Maul in Season 4 (Canon Editor, when does he get his normal legs back?)
The return of Satine in the new Mandalore arc in Season 5 (I haven't seen it, but I know what happens)
The final arc of Season 5, when Ahsoka leaves
The Yoda arc from the end of Season 6 (I haven't seen this yet either, but I heard it's great)
The last four episodes of Season 7
And then I need to figure out which essential episodes of Rebels to watch, to wrap up Maul's arc and to get resolution on the Vader/Ashoka relationship. (Sadly, it sounds like Asajj Ventriss' story is wrapped up in a novel rather than on any show, which is why I didn't focus on any episodes with her.)