If we're ready to talk spoilers, here's some stuff to chew on...
The big battle at the end where the heroes showed up is the coolest thing I have ever seen on film. Amazing stuff. I did not catch it on the first viewing, but apparently one of the portals has the Ravagers pop through with a gun-toting Howard the Duck. I cannot get over how cool it is seeing Ant Man as Giant Man in these battles, stepping on guys and punching the Leviathan. Cap wielding the Hammer and Captain Marvel taking a headbutt from Thanos without flinching was awesome as well.
I was not surprised by IM's death - I actually thought it would happen in Infinity War. But I was surprised how emotional that scene and the funeral were. I don't think there was a dry eye in the theater and that's saying something for an action flick based on costumed superheroes. I was less impressed with Black Widow's ending. She always seems to get the slight somehow, though I won't be surprised if they find a way to bring her back. Alternate timeline, trapped in the Soul Stone Universe, whatever. Maybe she's hiding out in the Negative Zone waiting for the new Fantastic Four to find her. Lost in the bigger storyline, it was nice to see Stan make his final appearance as well, and fitting with the end of Phase 3.
Marvel does such a great job laying out the potential for subsequent flicks. Loki disappears with the tesseract and likely figured out the heroes were time travelling. That's dangerous. Thor pairs up with the Guardians? Falcon's getting cap's shield? All great ties back to the comics for potential future plot points. I read "Falcon and the Winter Soldier" will be a new ongoing live action TV series, so looking forward to the continued banter between those two. Time travel always has consequences - will the divergent realities bring Kang the time traveler into Phase 4? Visions's stone is gone, but Wakanda was mapping his conscience before he died in Infinity. His color even drained, so the stage is set for the albino Vision to make a return and drive the Scarlet Witch insane.
My favorite setup was the potential for the Young Avengers. Hawkeye's daughter was looking like a great alternate for Kate Bishop's female Hawkeye. Cassie Lang is old enough to grab some Pym Particles and become Stature. Morgan Stark can carry the torch as a new Iron Man Lad Girl? Even young Monica Rambeau was set up in Captain Marvel, the secret identity of Photon (a woman who can turn herself into various forms of light). Spidey's still young enough to join up as well if they want to go that route. I'm expecting to see the new Phase 4 baddie set up in the end credits of Far From Home.
I'm also curious to see if they stick to the core timeline/reality. There is an alternate reality where Thanos never gets the stones, since he jumped into the main timeline. That means no Gamora or Nebula, but Widow, Vision, Loki, Heimdal, etc. are presumably still alive. I doubt they will go there anytime soon, but lots of potential to bring back Cap or Iron Man from another reality later on if they want to. How cool would it be in another decade to have older Iron Man and Cap time/dimension jump back into the storyline?
Overall, I give this a solid five out of five stars. You have to search pretty hard to find faults with this movie and the payoff for lifelong comic fans was well worth the wait. Bring on the next 22!