Most of my complaints about it have been mentioned before, but I'll reiterate...
Poor casting
Poor directing
Poor character development
So-so writing
I considered the possibility of doing it as a series.
I might have worked if someone like Jon Favreau was the writer and/or director. It would have pushed them into a different dimension relative to funding of talent, too. Rather than a ton of recognizable actors, they cast with talented lesser-knowns.
In the same vein, when considering a series, I lamented, "Holy crap, I cannot imagine dragging this out to 10 episodes." However, different writers, directors, and actors would have made it more consumable. Moreover, you only have to suffer through 45-50 minutes on a "weak" episode. The worst of the Mandolorian or Boba Fett episodes provided character development value, context value, etc. Each Eternals episode could have had a standalone thought/idea as needed.
At some point, someone inside Disney gave the green light to a 2+ hour theatrical release. At that point, they established an acting budget and target cast that would appeal to darn near every demographic. Gemma Chan is an attractive person and has performed well in other roles, but she was stiff and uninspiring here. Jolie is an overrated actress and just did not seem to fit. As noted, Madden as Ikarus - meh. Nanjiani - with each smile/smirk I saw Silicon Valley. (BTW - he is cast in the upcoming Obi Wan Kenobi seris).
I do believe it opens up another door in MCU and could be the springboard for some new/different storylines. Let's hope they clean it up a bit and do a better job at casting and directing.