Rob Bell is, at best, standing on very shaky theological ground. I am no theological giant myself, but a lot of the stuff Bell says is just BS, and it contradicts scripture. That's the easiest way to put it. I don't have any exact quotes because I don't have a copy of either of his books with me, but if you've read them, you know what I'm talking about.
Here's an example:
-In Velvet Elvis, Bell talks about the story where Jesus walks on the water and Peter walks out to him. Bell says that Peter did not lose faith in Jesus, rather Peter lost faith in Peter, which is why he sank in the water. Peter lost faith in his own ability to walk on the water.
The story, as told in Matthew 14:22-37, is that the disciples are out on a boat and Jesus walks out to them. At first they think it's a ghost, but Peter calls out and says that if it is Jesus to have him walk out to him. Jesus tells him to come on, and Peter walks out, initially believing in Jesus and his ability to let Peter walk on the water. Then, Peter sees the wind and the waves and gets scared and sinks, and Jesus grabs him and takes him back to the boat. Peter took his eyes off of Jesus and he sank. This is in sync with John 15:5, which says "I am the vine, you are the branches." This verse ends with the phrase "Apart from me you can do nothing."
According to Jesus, apart from me you can do nothing. Yet, according to Bell, Peter had the ability in and of himself to walk on the water. Something has to give right there.
-Bell, and the Emerging Church in general, aren't too crazy about the thought of Hell, either. Hell, or the eternal separation of man from God, is a real thing. In Sex God, Bell talks about Hell but he only defines it as a situation that we create on earth when we are mean to people. Again, the Bible talks about Hell, a real place where man is separated from God for eternity, several times. I don't know if Bell personally believes in Hell or not, but it's a pretty significant thing and it's like a heresy by omission or something. I don't know if that's a real term or not, but it's omitting part of the truth, and that is not a good thing.
There's some other stuff too, but I don't feel like looking it up. I don't think people should avoid Rob Bell and his books/videos/sermons, because I do think he says some good stuff occasionally, but people who read him need to do some research on what he's saying and not just accept it for truth. He does a fantastic job of packaging stuff into a hip package that is aesthetically pleasing, and because we think he's so cool and his books are printed so uniquely, we don't look into it much.
But as with anything, scripture is the ultimate standard, and Jesus Christ is the ultimate truth. As far as I can tell, while Bell does say some good things, he has a lot of very significant points that simply do not line up with scripture.