What then, are the weaknesses in the evidence and reasoning that Ehrman presents?
He's actually a bit more credulous in his acceptance of the reliability of the text than I am--but not much. I don't give much credence to the concept of Judas Iscariot as a historical person. Ehrman's arguments work well with Judas as a literary character, but that gives more insight on what the early Christians thought than what Jesus himself thought.
But even if Judas is not a historical person, Ehrman's arguments are not fatally damaged. While the extent of Jesus' actual words in the Gospels is disputed, I can accept that we have some authentic words, and I agree with the mainstream position that what we have of Jesus' speech often used paradoxes and parables.
Where the argument is a bit weak is that one of the criteria of "authentic" words of Jesus is a lack of self reference. For example, Matt 16:13b: (paraphrased "What do men say of me as the Son of Man?")--this is widely accepted as a later theological addition, rather than an authentic statement. From what we see of authentic text, Jesus didn't put himself forward as the leader of the "Kingdom of Heaven"--he was simply a prophet _for_ the Kingdom of Heaven.
Part of that, I feel, comes from the history of prophets in Jewish literature. In Judaic culture, prophets were not kings--they had separate roles. This division of prophet from king
is a fatal flaw within Ehrman's arguments.
I'm not expressing this as clearly as I could--I'm not having the best of days today, so I'm not as coherent as I would like. Ehrman's argument is, to my mind, somewhat weak, in part because it seems he's relying on post-crucifixion views of Jesus, rather than on what Jesus actually said.