My immersion was slowly eroding upon the opening scene with Vega. It wasn't Vega's character or anything, I've actually grown to like him a little (though he will still never truly be the replacement Krogan Bioware intended him to be). Just the moment of "Hey, big muscle guy who I don't know but is clearly friends with me."
Ventchild's sappy death music as Bioware tried to bash me over the head with an emotional hammer took my immersion down a large notch (and hell, this was WAY before Starboy showed up, blech), followed by not being able to catch up and stop Eva before the predestined landing pad of shuttle-colliding destiny on Mars. Even as a Vanguard, who of all character classes, should've been able to catch her and promptly dispose of her.
It finally broke when the first dream sequence hit. The eavesdrop-scan-quests didn't help either.
Tuchanka was later able to help move me along, but just as I thought I'd be able to pull through, the Rachni popped up. While the timely arrival of Grunt single-handedly picking up a Rachni and throwing it off a cliff was awesome, the Rachni choice itself was laughable.
Kai Lame arrived. 'Nuff said.
And then we had the silly Rannoch arc with its useless turret section, and whose end boss made me double check to see if I was hallucinating and just imagining Shepard using the Hammer of Dawn on a destroyer where Marcus Fenix would normally be hammer of dawning a Lambent Brumak.
And then Kai Lame hit again...and then he sent his silly little email.
Needless to say, by this point I had utterly convinced myself that ME3 could not get any worse than Kai Lame. Then the crayola ending came.
AAAAAAAAARRRRGGGGHHHHHH!!!!!!!!!!