Rob Bell is identified with a Emergent Church despite not self identify as a member of the emergent movement. However, he tends to advocate many of the ideas of that group. Case in point, Love Wins, which came out last year, was hugely controversial, since it was putting forward a view on the nature of hell which is not held by many evangelical Christians. Many had accused him of being a universalist. Prior to the publication of this book, Jesus Wants to Save Christians: A Manifesto for the Church in Exile, came out, which is many would classify as an extremely liberal gospel interpretation (he uses the gospel to criticize US foreign policy for instance). These types of views are extremely controversial to many evangelical Christians many of whom tend to (justly or not) fall on the conservative side of the political spectrum.
Also he wrote a book called, Sex God: Exploring the Endless Connections Between Sexuality and Spirituality, which having read it, I can say is not nearly as scandalous as the title suggests. But, the point is, Bell meant to stir put feathers by publishing a Christian book with a title like that, and that tends to be his modus operandi.
Hope that helps.
Update, a little bit more on those that believe Bell to hold to heretical beliefs:
On February 26, 2011, John Piper tweeted "Farewell, Rob Bell" and linked to an article accusing Bell of being a universalist with his new book "Love Wins". The implication was that Bell had strayed so far from the Christian camp as it were, that he no longer considered Bell to be part of the Christian community (that's my personal take on the meaning of the tweet).
The real issue here is not whether or not Bell was enunciating universalism or not (Bell in a sermon the week after publication said "I am not a Universalist" and made the same statement to various members of the press, for what that is worth), but what does it mean to be heretic.
My take is that if you take a look at his main critics, especially Piper and Chan, they tend to fall into the Calvinist camps, that not to say they all do by any means. But I think if you look into the heart of the controversy about Bell, who is an arminian by any measure, it may have more to do with justification than anything else. For Bell, we choose God, for Reformed thinkers, God chooses us. Beyond the specific ways in which each sides is interpreting scripture, each side holds fundamentally opposing theological views.
For more info on Bell critics, see Francis Chan's Erasing Hell, Mark Galli's God Wins: Heaven, Hell, and Why the Good News Is Better than Love Wins.