All of that is correct that game itself is good and in NORMAL situation it should get around 8/10 radius. Problem is, that it is NOT a normal situation.
As if you look at the game as a solo game with nothing else, game actually goes down to around 6/10. Outdated graphics, Problems with graphics (head spins, terrible camera angles in conversations on many occasions), only one mod for multiplayer with retarded servers, BUYING gear for real cash for multiplayer?... ye okey, to LINEAR for RPG with 0 choice importance (for an RPG decisions are useless, they are in forms will I bang this character or that one. Or will I get this war asset instead of this one). THAT is actually biggest problem. 0 choice importance. An RPG should have some sort of it was needed to do that, in this game it's not. I wouldn't mind it that through entire game they didn't make it look like everything I gather matters a lot.
And now back to 8/10. Yes, game cannot be looked as a solo game, it is a trilogy. Trilogy of Shepard and characters we learned to love and hate. We got the goosepumps through the entire trilogy with so many OMFG moments. I still feel the OMG feeling when I talked to Vigil and found out the plan of Saren and Sovereign. My eyes almost poped out. To give the game it's deserved 8 I have to look at the entire picture, of all the details, all the props added into the game, all the changes from choices in last 2 games. As that, yes I can easily give game 8, even 9 out of 10 for bringing the feeling and keeping all the characters in their own element, their personal it's THAT character feeling.
Now, why does game insta get low scores? As I cannot look at it as anything other then as a disappointment. Game to get high scores requires players desire to replay the game + long play time consumption. Sorry, this game DOESN'T have replay value as no matter what it comes the same, no different choices as it is linear. I can't even get to know what those few choices that are different actually meant as final message is Buy extra DLC and nothing about all the galaxy and characters I loved. So in forms of looking it like that, YES game actually deserves even lower scores then as looking it as a solo game which is a 6, so overall score is actually around 3-4 out of 10.
So no, I am not surprised at low scores and IGN can go kiss their EA masters asses for some more free stuff from them.