While Einstein contributed extensively to the foundations of Quantum Mechanics, he refused to accept the philosophical ramifications of the whole field. His objection is probably best summarized by his famous quote: "My God does not play with dice."
If you think about it, the thematic differences are pretty profound. General Relativity implies that we can understand, describe, measure, (and potentially control) everything in the universe. Quantum Mechanics implies that we can't.
What I find interesting is that the Western way of understanding the world is very much like General Relativity. If you can't measure it, it doesn't exist/matter, which has led to a number of problems. Could some of our more systemic problems today be described metaphorically as stemming from our refusal to admit to this contradiction?