Defend this. What issues do you have with the game before Priority: Earth?
The entire intro sequence with Shepard and Anderson on Earth is hackneyed and contrived, especially since the game assumes you played Arrival, a bad DLC, and makes very little sense without it
The Mars sequence is rushed and a way to introduce a magic off switch, which they can build despite not knowing what it is, but describe as doing something with energy, which is basically the fundamental description of a machine
The EDI/Joker relationship is garbage
Javik is DLC
Kai Leng is an awful character that the game desperately wants you to feel is some powerful antagonist rival but just comes off as an annoying prick protected by hilariously transparent plot armor
The character side stories often feel like they exist merely to conveniently finish off character arcs or give former squadmates who didn't make the party cut this time an excuse to not be here the most important thing ever at the end of the universe
Personally this is a massive nitpick but I hated how Mordin just completely made up his mind and started working for a genophage cure off-screen between games and also I like his default death line rather than the one that most players will get (where he hums Pirates of Penzance if you got him to sing it in 2, which feels like a fanservice meme inserted into his character arc's finale). this is super nitpicky i know
Cerberus once again completely changes form, this time into a super army of infinite dudes even though you were told they had no resources left after Project Lazarus to revive Shepard in 2
The Galactic readiness thing is really half-baked
The weird Pac-Man thing on the galaxy map
There's all those missions where they drop you into the multiplayer maps to pick up a goober but unlike the multiplayer which is fun these are boring filler
Half the sidequests are boring filler where you overhear someone saying "well we lost our space stick on planet gobulon v when the reapers attacked" and then shepard magically teleports the bafmodad into the Normandy by scanning the planet and then walks up to the alien and is like "hey I found your thing" and they're all "wow great, NOW i'll help stop the apocalypse"
Certain plot arcs like destroying/preserving the baby reaper or the rachni ended up barely mattering outside of a handful of GR points
In contrast to 2, which had a lot of varied scenarios, the plot of 3 necessitates that nearly every single mission is set on a tech base where a bunch of Cerberus guys jump into with rockets, or a reaper-invaded rubble planet covered in husks. the only major deviation from this is the Rannoch missions
I really hated the writing for Rannoch, which ended up just making the quarians even bigger assholes than they seemed, and the finale of the geth subplot is that all of the cool unique things about them as a gestalt synthetic species that wants to exist on its own terms are thrown out the window so they can become generic robot individuals; Legion's death, while thematically necessary, was written super awkwardly (I can't copy and paste my code so I have to cut and paste my code, which will work instead... I have to go now, my planet needs me); afterward EDI gives this incredibly insulting explanation about how the geth having a form of consciousness that deviated from organic standards was objectively inferior and incorrect so it's good that they conformed and destroyed their unique identities. fuck off