If there is a difference in the two testaments I would put it down to a matter of expediency - God's judgement was often immediate in the Old Testament whereas in the New Testament the judgement is promised to come one day in the future when our eternal lives are at stake. That doesn't mean that God isn't less judgemental, and if you think about it it could even be said to be worse since our eternal souls are at stake - that was not at stake in the Old Testament.
Whichever way you look at it, I find the stereotype of God in the Old and New Testaments to be utterly false.
I agree with you on God being both loving and violent in both books but he does seem more harsh to me in the OT and I think this is so because it was basically Yahweh Version 1.
Jesus came along and wanted to upgrade him and improve him and so decided to depict him as a little nicer while still maintaining some of the madness and authority E.G. Ananias and Sapphira, the Fate of King Herod and the entire book of Revelations. Those are the only major accounts of God killing or planning to kill people in the NT whereas it's clear the OT contains a lot more such as The Flood, Sodom and Gomorrah, Onan, Lots wife, The first-born's of Egypt at the hands of the Angel of Death, a few plagues wiping out a thousands, kids being mauled by bears and a few other accounts but you get the idea...maybe he just had more reasons to rain down his 'justice'.
Of course you could just as easily say that God was slowly revealing himself and just by chance, never had to be as rough on the NT generation; Kinda' like conditioning us until we learnt, first he had to be tough and then when our rears had been spanked enough, they'd be nice and conditioned to gods big hand...until Revelations comes about that is.
I'm not saying God has to be fair to everyone, he doesn't...he's God after all, he can do what he likes as he has shown many ties before hand.