Spoilers, obviously. If you haven't seen the movie yet, probably skip this.
Things I Didn't Like
As with The Force Awakens, while I appreciate the new creative teams' desire to break new ground, I would have liked to see more familiar/existing aliens sprinkled into crowd scenes (we did get one Twi'lek, apparently a relative of Bib Fortuna, in Saw's group, but that's about it, aside from a new Mon Calamari admiral (who, paradoxically, I'm glad wasn't Ackbar). During the Battle of Scarif, there was a large furry alien firing from a Rebel gunship, and at first glance, I thought it might be a Wookiee. Turns out (per the Rogue One visual guide) that was a new alien from a heretofore unmentioned species; why not just have it be a Wookiee?
Ditto the lack of previously-mentioned planets; of all the new planets introduced in the movie, not one of them could have been a planet mentioned before, either from a canonical or Legends source?
I knew the movie wouldn't have an opening crawl (and that's fine), but I did miss not having the "Star Wars" logo flying away from the screen after "A long time ago, in a galaxy far, far away" appeared. I guess they felt that logo was too tied to the bombastic opening theme which (again, understandably) they weren't going to open the movie with, but I still missed it in a way I didn't miss the crawl.
Early in the movie, the name of each planet was listed on screen; no other Star Wars movie has done this, regardless of how many jumps it made from planet to planet, and the audience followed the plot fine. Seemed unnecessary and not-Star Wars-y.
I have no problem with wanting to end the movie with Leia aboard the Tantive IV, Death Star plans in hand, but having her actually be physically present at the Battle of Scarif seems unnecessarily complicated (and badly setup), just so they could have the scene of Vader almost stopping the plans from getting to her ship. I guess the idea is that Bail leaves Yavin 4 (after telling Mon Mothma he'll send Leia for Obi-Wan), goes to Alderaan, dispatches Leia aboard his ship to get Obi-Wan, and then en route, the call goes out for every Rebel ship to go to Scarif, and Leia responds, ending up docked with the Mon Cal cruiser (for some reason) before flying off with the plans. All of which is fine, I guess, but in the interest of keeping the Leia cameo a surprise, the audience has to intuit a lot of the steps in that sequence for themselves (having Threepio & Artoo observe the Rebel force leaving for Scarif also complicated things, as it compressed the timeline - Bail had to still be on Yavin 4 at that point in order for the droids to be aboard his ship when Vader attacks in at the beginning of A New Hope; in principle, I'm fine with them being in the movie - it keeps their streak alive - but there was probably a better place for their appearance).
Things I Did Like
Tight continuity - for the most part, this lines up remarkably well with A New Hope. I watched the beginning of that movie again this past weekend (thanks to TNT running the saga films off and on pretty much all weekend), and the climax of Rogue One matches beat for beat the story detailed in the first two paragraphs of the opening crawl (striking from a secret base, Rebel spies, first big victory, etc.). Plus, we now have an official, canonical explanation for the Death Star's fatal flaw, and the Battle of Scarif makes a handy explanation for why more Rebel ships weren't on hand to later attack the Death Star.
Everyone died - I guess Jyn's not Rey's mom after all (not that I ever really bought into that theory)? For a movie whose ostensible theme is the power of hope, this is without a doubt the darkest Star Wars film, with all the main characters dead by the end. If A New Hope is the dawn of the Rebellion, than this is definitely the "always darkest" moment.
They swerved dangerously close to shoehorning in a Jyn/Cassian romance, but ultimately held back just enough.
Darth Vader cutting loose - due to the limitations of effects and costuming in the Original Trilogy, we never really saw Vader fighting in an intensely physical manner. So it was a treat to see him in all his armored glory slicing-through, Force-choking and Force-throwing Rebel troops in full rage.
No wonder George Lucas liked this movie: he finally got the Darth Vader lava castle he had in numerous drafts of the Original Trilogy scripts that always ultimately got cut (also, I'll set the over/under for the number of sites speculating that guy who fetched Vader when Krennic arrived, Vanee, is Snoke at about 25, and I'll take the over). According to the Art of Rogue One book, the planet the castle is on is actually Mustafar, site of Anakin's maiming in Revenge of the Sith, which also appeared at the end of the first season of Rebels.
Speaking of Rebels, during one of the later scenes on Yavin 4, a loudspeaker annoucement can be heard calling for General Syndulla, presumably a reference to Rebels Hera; hopefully this means the show will continue to move the main characters closer and closer to the larger, galaxy-wide Rebellion of this era (my buddy also said he saw Hera's ship, the Ghost, during the Battle of Scarif; something to watch for next time).
Death Star porn. Lots of great Death Star shots in this (and some good Star Destroyer stuff too). I also love that they managed to recreate A New Hope's aesthetic of having most of the Imperial officers played by stuffy British-looking old white guys.
Given the amount of dialogue he had to give, the CG Zombie Peter Cushing was pretty remarkable.
Ditto the inclusion of Red and Gold Leaders via cut New Hope footage.
Being a movie in which the central MacGuffin turns out to be data, this did an admirable job of striking a balance between the thoroughly-analog technology used in A New Hope and a modern audience's intuitive understanding of how data is stored and transmitted nowadays. Rogue One couldn't just have Jyn uploading the files into some kind of galactic Cloud, because it would contradict the tech of A New Hope, but it couldn't also just resort to 70s-era analog technology, because it would take away from the sci-fi, vaguely-futuristic feel of the universe for current audiences.
It took me a few viewings to fully appreciate the Force Awakens score, but I liked this one on first viewing (especially what I guess is the main "Rogue One" theme that seems to be building to a variation on the classic Force/Luke theme, but doesn't quite get to the triumphant moment in that cue), even without John Williams and much in the way of recognizable existing themes (though I'm not so sure it will hold up as well as a standalone listening experience).