He was a proponent of hidden variable theory, which tried to reconcile quantum mechanics with determinism, famously saying "God does not play dice". People often say that hidden variable theories were proven impossible, and thus Einstein was proven wrong. That's not quite true, and only local hidden variables have been ruled out.
It also disparages his contribution to the scientific discussion to just state that he was "proven wrong".
Bohr's argument in the discussion was a bit of a mess and I couldn't pull anything out of his rebuttal to EPR other than an assertion that QM behaves the way it does and not to pay any attention to the man behind the curtain. Its a very philosophical argument with very little scientific content and he just proposes that the QM math is correct because its correct, as far as I can tell.
EPR made a logical cogent argument. It was based on the philosophical principle of the locality of physics. They translated that into the mathematics of Quantum Mechanics and proposed a simple experimental test. Later that was refined by Bell and tested experimentally by Aspect and others. It was the Einstein-Podolsky-Rosen paper that laid the groundwork of how to test the non-locality/hidden-variables of QM though.
EPR moved the scientific discussion forwards much more than Bohr did, but it turns out the test they proposed showed that the position they favored was incorrect.
Also Einstein was arguing first and foremost that physics must be _local_. That's in opposition to the "spooky action at a distance" bit that he didn't like. Since local hidden variables are ruled out then he really was proven "wrong".
TL;DR I think Bohr's argument is rubbish, and Einstein's is solid, but the Universe is a bitch and doesn't care...
