Of course there are significant differences between the G-d of the Hebrews and the Christian God. In Western countries many (though not all) atheists are rejecting the Christian concept. To illustrate how different the two concepts are:
The Jewish G-d evolves as does the Jewish people’s understanding of G-d.
This is the opposite of the Christian view that their Trinity God does not change, is unchangeable, and has no need for change. It also allows Christians to claim as heretical anyone who wishes to introduce an evolved view of their God.
In Judaism, G-d is unknowable.
This is also very different from Christianity. Through Jesus Christ, say Christians, God can be understood. Jesus, they claim, is the perfect reflection of what God is. If you know Jesus then you know God.
To the Hebrews, G-d welcomes arguments from his servants.
Christians believe that God should be listened to and obeyed without question, but from Abraham onward Jews are known for their very unique relationship with G-d, which includes things like debating, bargaining, and like Jacob (whose name was changed to Israel for doing this), wrestling with G-d. This is why Jews are known as the children of “Israel” and not “Abraham.” A relationship with G-d is to be explored and dealt with like any other relationship for Jews, with no need or desire to surrender just because the other person you are dealing with is your Creator.
G-d is…
While we just acknowledged that the Creator is “unknowable” to Jews, there are few things G-d “is” that can even rub Christians and others the wrong way.
To Jews, G-d is not recognized as a “he” or “she.” In fact the Hebrew pronoun for G-d, which unfortunately gets rendered as “he” in English in many Gentile translations, is actually a neuter pronoun. That is why you often hear Jews say “G-d does things on G-d’s own terms” instead of “God does things on his own terms.”
Some Jews believe that G-d is the literal Universe, while other Jews believe G-d contains the Universe while at the same time being infinitely greater. Some say that all conflicting concepts about G-d can be true at the same time. But in the end all agree that this does not change the fact that in the end G-d is both ultimately unknowable and even unnamable.
To Jews, G-d is not necessarily perfect, complete, or all-loving.
This may shock many, but it is true. Some Jewish theology suggests that G-d created humanity in order to challenge G-d to become better at being just, loving, and perfect. This is also the reason why the concept of G-d has changed over the centuries among Jews. The ideas presented in Biblical narrative, which Christians view unchangeable, Jews consider to be sometimes primitive and infantile and incomplete views of G-d. The idea of G-d being merely a monotheistic imageless Deity is “so 4000 years ago!” G-d has acted in ways in the past that G-d would never act like today or tomorrow. And Jews once held ideas about G-d that they would never accept as applicable today.
Jews can even be humanistic or secular which can include atheist and agnostic convictions or responses to the Jewish G-d concept. That is one thing that cannot be compatible with Christianity.
Disclaimer: The above is in no way intended to suggest that any who currently claim to be atheists are not because they have not fully reviewed what other religious concepts completely entail. On the contrary, the above is meant to demonstrate how the Christian view has often dominated the Jewish view to the point that many reject the Jewish concept of the Creator without realizing there are striking differences.