Comment #92 deserves some attention.
1. It affects the single player game mechanics. The maps in ME3 would have been capable of handling 3 individual players fighting together. As far as difficulty of your opponents, that can be easily rectified by simply throwing in a few extra opponents for each additional friend with you. No friend, no extra baddies.
2. Friend isn't a good reason. Well that's just wrong. If people want to play games with their friends and if Bioware wants to make games for people to play with their friends, then it's a good enough reason for them to make those games.
3. Ability to pause. There's no reason for that to go away. Yes, they would have to tweak the powers to ensure one player's power doesn't time out while the game is paused, but otherwise there's nothing to it. Yes, if you choose to have friends play with you, they're gaming style is going to affect yours. I liked to pause and throw out a five or six grenades one after another. The sniper guy is going to have to wait for his shot.
4. Fun vs co-op. Co-op is fun for some people, Co-op is more fun when you play with actual friends and not strangers. Co-op has some fun for some people when they play with strangers, but not for me. I stick with people I know. If the priority is to make the game more fun, then developers will at least look at co-op or multi-player.
There was another post that had an objection about standing around while someone else flirted with an NPC. Why would you stand around and watch that?. They should be capable of letting you run around the ship, or home base on your own; only requiring you to be together and agreeing on where to send the ship to next.
Several other posts I think have hit most people's objections. After the ME3 fiasco about requiring multiplayer initially to raise the readiness no one trusts Bioware to not repeat the same mistake. Screw over the SP game by forcing some sort of co-op or multiplayer on the game. That is a reasonable objection to even considering co-op. It is also a valid reason for Bioware to try again. Bioware saying, "we didn't do so well on the last one, let's learn from our mistakes and try again. Let's get it right this time."
And I'll reiterate dialogue choices can work. A or A+A gets A. B or B+B gets B. A+B gets C.