I can celebrate the message of love and hope as expressed throughout the New Testament but find it difficult, almost impossible, to identify with God as described throughout the Old. How can God change so much? In the Old he is cruel, jealous, vengeful and murderous (there is plenty of evidence to back up all these statements); in the New he is LOVE. How can we square this? What changed?
There are several ways to "square" this. They do depend on not regarding the Old Testament as absolutely and literally "the word of God" - i.e. dictated by God to Moses, the prophets, etc.
One is to regard the writers of the Old Testament as reflecting the assumption that if anything happens - good or bad - God must be behind it, thereby to God is atttributed all manner of human evil, and all manner of good things (though often "the good" is at the expense of one's enemies, taking the argument full circle).
Then there are arguments that some of the language used is less horrific (in a particular context) than we interpret it.
There are many "ways out", but whether, even cumulatively, they get near an easy reconciliation of Old and New Testament is difficult to assess - especially when Jesus seems to see the Father as rather more loving than some of those texts imply.
I had a quick look at some online resources. http://enrichmentjournal.ag.org/201203/201203_034_Good_God.cfm
seems a fairly detailed attempt at an answer - better than I can manage off the cuff anyway.