How has Mass Effect 2 'dumbed down' the series?

[NinjaDeathSlap]

The only thing that I felt was worst in ME2 than in ME1 was the story... It was way too mankind-centric.
In the first game, mankind was the new kid in town. We weren't the heroes, we weren't the best and even when compared to the asari or the turians, we were like kids in a playground. We were newcomers and even if mankind as a whole become extinct, the galaxy would still go on as normal. Threads like the Reapers were some kind of eldritch abomination, too big, too powerful and too unstoppable to even dream of fighting alone. Then, in ME2

Spoiler

it turns out the Collectors are after the humans (and humans alone... not the turians, asari, elchor or any of the tens of sentient species in the galaxy), following orders from the Reapers to gather or kill us so they can turn us (just us, apparently) into Reapers... no other specie was in immediate danger. In other words, they changed the role of mankind from one of the characters in the play to the main protagonist,

which makes the motivation for characters like Garrus, Samara or Mordin a little harder to believe.

Mordin is a scientist/spec-ops soldier, so the combination of mystery and adventure excites him.
Samara owes you a favor after you helped her with her investigation on Illium.
Garrus is your friend, he's looking for a way out of Omega anyway, and he already knows about the threat the Reapers pose to everyone, so of course he's going to follow you.
(as for the other non-human allies...)
Thane is on a quest for self redemption, and saving an entire race from genocide is a pretty redeeming feature.
Legion is fascinated by you and realises you are the only one who can warn the galaxy about the Reapers as no-one trusts the Geth.
Tali already knows you, is in love with you, and you do save her ass on Haestrom.
Grunt is just in it for the killing, although he does respect you once he sees you in action.

Which means every other non-human is there because of YOU or because they were hired. Its not that they are not motivated, but it makes it look like only humans are concerned and endangered by the Reapers (who, by the way, doesn't appear until the end of the very last mission).

Well, I'd still argue that Thane, Mordin, Garrus and Grunt are there for their own interests at least partially. But they wouldn't work for you if they didn't need you or didn't respect you, same goes for the first game.

I've never really seen it as too 'human centric'. Sure, they do have a bigger presence in the second game, but after the events of the first how could they not? Maybe it's because I've always adopted the Paragon stance, imagining my Shepard to be an advocate for everyone, not just humanity.

It is worth remembering that the Reapers ultimate objective is still the extinction and re-purposing of ALL advanced species in the Galaxy. Using the Collectors to remove Shepard and the rest of humanity from the picture is just a warm-up to the main event, which is kind of what the second installment of a trilogy is supposed to do anyway.

It's still the case in ME2 of over half your team being non-human, even when you count in Zaeed and Kasumi, so I'm not sure it can be accused as being too interested in just showing everyone how awesome and important humans are.