I have never ever understood people who say this. I like the first. Enjoyed the second and have found the others boring but it had nothing to do with the story. The story was rubbish all the way through. Have to say the co-op was good though.
It´s not so much the story of the campaign, but the setting and the BIG story behind it.
The Concept is not entirely new, I grant you that, but it´s really fascinating.You should try and read some Halo books, there are really good among them.
It is a well- thought out Sci-Fi setting as it is rarely ever seen in games
Also, I dont think the gameplay is meh.I said it didnt change much and is not really innovative; For me, it doesnt need to be.The gameplay is awesome as it is, why would I want it to change?