as you implicitly mentioned, the question of what is acceptable performance cannot be answered objectively but has to be defined by the standard. For the current LTE-A specification, this is given by a target Block Error Ratio (BLER) of 10%. This means that on average, one out of ten times, the decoding of a received codeword fails. To achieve this, the transmission parameters (CQI, RI, PMI) are adapted according to the channel state by the feedback algorithm.
However, as explained, the region of operation is defined in terms of BLER and not in terms of the BER.