He's not really supposed to be a character, per se. He's more of a plot device, a MacGuffin to get the characters into the situation they're supposed to be in.
You're not wrong on that one, which makes it sad for SAO that he makes for a more entertaining plot device than Kayaba, who doesn't do anything other than kick off the plot and then end it with a fight.
IMO, it would actually be lazier writing to have him say that he was bored or something like that. "I forgot" seems something a psychopath would be more likely to say (although the series seems to have differing opinions on the sanity of the character in the second arc).
You don't know much about psychopaths do you? Most of them are simply lacking in empathy and emotion, without any sadism or malicious intent, but the ones who do messed up stuff like go on serial killing sprees do so because they are bored and they find such acts the only things that really stimulate them. All are quite intelligent, and while they do sometimes suffer from bad memory (due to the boredom) they would certainly never forget their entire reason for killing thousands of people in a video game.
The in-universe defense for this is that Kirito is a strong character in a dog-eat-dog world, and that they are attracted to his strength and such, but if we're being honest, it's really just an anime cliche (it's really only three woman that do this in the course of the show though, to my knowledge).
That in-universe defense sounds pretty sexist... I didn't even think of it that way until you just now put it like that, I just thought it was stupid wish-fulfillment crap. Well, the show loses even more points now! Oh, and counting the females who want to bone him, there's Silica (a loli girl... wtf) Lisbeth (who has no notable personality) Asuna (textbook tsundere) his sister/cousin (how the flying fuck would his own sister/cousin fall for him when all he does is sit on his ass playing video games?! I mean, at least the other girls don't know that he's such a bloody loser, she has full knowledge of it) and in Gun Gale Online there's Sinon (haven't actually read that so can't actually judge...)
The rest of what you said is completely subjective and not something I can argue against, because especially in the first arc the "plot" doesn't really kick in until the love interest starts - everything prior to that is world building, character development, and related things. Different strokes for different folks.
Well, I suppose the last part is subjective, though I don't really see how anyone can stand boring stories like helping the loli girl revive her dead virtual pet, or cooking, or fishing, or adopting a kid one day after getting married when there are more interesting things to see in a series about living in a video game (I could understand if the side stories were good, and not blatant otaku-bait). Particularly when a pivotal part of it, the romance, is so contrived, rushed, and generic that it could have been made using a chart with bullet points on "How to make a romance resonate with otaku/gamers".
