Great question! (Sorry it's taken me a while to get back to you.) I'll just let Dan and Jeff speak for themselves since I obviously don't entirely agree with their theology anymore. Instead I'll take a stab at my own answer.
The first thing I'd say is that I understand your concern about the difference between the God of the OT and the God of Jesus. You're not the first person to see this distinction (cf. Marcionism). And I can't say that I have all the answers. There are still plenty of parts of the OT (and the NT for that matter) that don't make sense to me and some that really disturb me. I don't think I'm completely capable of justifying all the ways of God throughout the Bible (but I'm not sure I should be able to - that's kind of what I mean by him not being a tame lion - if I understood everything about him I'd doubt that he's really God).
But anyhow, to your question. It's easy to caricature God in the OT as "Murderous, vindictive, jealous, vain and capricious", but I think that may result simply from a surface reading of the text without really delving into the meaning of the stories. Personally, I didn't really start to understand the OT stories until I started to understand injustice and how much God hates it. As I became aware of all the injustices in our own world - from sex trafficking, to modern slavery, to exploitation of the poor, to gender related violence, to genocide, etc., etc. - I started to realize that most of the time when God seems so angry or violent in the OT, it's because he's angry at these kind of injustices. That's the tension throughout all of scripture is that God is both a God of love but also a God of justice. He fights for the cause of the oppressed, the poor, and the abandoned - so it's natural that he would seem a fearful and vindictive God to those of us (like most of us in modern Western society) who more closely identify with the wealthy oppressors. But try reading the Bible through the eyes of a Latin American peasant, or a Sudanese refugee, or a Southeast Asian sweatshop worker - suddenly the God of the OT seems like a welcome figure of liberation and justice.
I'm very serious about this. Until we read the Bible through the eyes of the marginalized and oppressed, I don't think we'll really understand it, since the vast majority of it was written to a marginalized and oppressed people. Until we learn to get very angry about the oppressions that still go on in our own world, I don't think we will be able to understand why God gets so angry too.
(Just one example, try not to read the Exodus as a story of God smiting the innocent Egyptians. Instead read it from the point of view of an escaped slave fleeing for your life against the might of the most powerful empire on earth at that time. Put it into our own times. What if the story was about God standing in the way of the Nazis as they were about to perpetrate the Holocaust, or in the way of the Sudanese nationals as they slaughter the people of Darfur? Suddenly God's violence seems a little more justified - after all we don't fault the Allies for resisting the Nazis.)
But I think you probably get my point. Again, I'm not saying I can explain away every disturbing thing in the OT this way. But using these new lenses, the lenses of the oppressed, a lot of things about the OT have started to become a lot more clear.
BTW, I think part of the point of Jesus' message - i.e. the way his God is still connected w/the God of the OT - is that he comes to offer mercy not just for the oppressed but for the oppressor as well, because Jesus points out that the oppressed too often turn into the oppressors once the tables are turned. This is the story of Israel in the OT - everytime God liberates them from one oppressor, they turn right around and start oppressing others - so God smacks them down again. In the NT the Jews are again looking for liberation from their oppressors (Rome) but Jesus' message is that they are really oppressing themselves - through the exploitative Temple system they've set up, through their racist and exclusionary religious rules, through their own inclination towards violence and hatred. Basically what Jesus says is "This whole time you've been praying for God to smite the oppressors and bless the oppressed, but what happens when the oppressed are their own oppressors? What if the problem is that all people, oppressed and oppressors, have the same inclinations toward violence and evil and oppressing others? What if the only solution is to break the cycle of revenge and offer forgiveness and peace to both sides?
Again, this is why Jesus was crucified - the Jews were looking for just another liberation from their external enemies, and Jesus said, no, first you need a liberation from yourselves, from the enemy within. That's not what they wanted to hear, so they killed him.
Anyhow, sorry for the long answer, but this is stuff that I've just been discovering myself recently, so it's fresh in my mind. I hope I've made a little bit of sense and perhaps even answered some of your question.