I honestly don't think Ebert really likes mindless action flicks, which is what this is. You aren't supposed to terribly worry about the plot, although the plot does have some interesting twists and turns in it. I think what usually makes a mindless action film good are the characters. They have to be interesting and/or fairly likable. You know the movie Shoot 'Em Up? That movie would've been awful if the main character was boring (although he was supposed to be somewhat dim character-wise) or if Paul Giamatti's character was less than stellar.
But if you read his review, you'll notice that he complains about some of the "silly action sequences" and tries to say things like "that would never happen!" No shit, Ebert... it's called stylized action for a reason. It's usually meant to purely entertain, not meant to be more realistic (like the Bourne trilogy).
I usually prefer reading Ebert's reviews over anyone else, but I can easily spot if he's not being objective. He wasn't very objective in his review of A-Team just like he wasn't with the movie Kick Ass. Although, the thing is that reviews are typically designed to be opinionated pieces, and you may not agree with some of their opinions

. But it would be nice if they tried to judge a movie on its merits regardless of whether or not it clicks with you.
What might be kind of surprising, is that I've used movies like Shoot 'Em Up as an example of stylized action, but Ebert actually liked Shoot 'Em Up:
http://rogerebert.suntimes.com/apps/pbcs.dll/article?AID=/20070906/REVIEWS/709060304/1023
But to give you an idea of why he hated a movie like A-Team, I'd take a peek at Taken's review:
http://rogerebert.suntimes.com/apps/pbcs.dll/article?AID=/20090128/REVIEWS/901289987
He rants for half of that article about the premise of the movie and how he goes on a rampage throughout France.
Ebert just doesn't get a fun action flick.