Why isn't the Bekenstein-Hawking Entropy considered the quantum gravitational unification? Based on the Bekenstein-Hawking Equation for Entropy, hasn't the relationship between quantum mechanics and gravity already been established.
 A: To add to Dilaton's correct answer:  The black hole area law is a result in classical gravitational physics.  It tells us something about the macroscopic behavior of gravity, but it doesn't tell us anything directly about quantum gravity.  It isn't even formulated in quantum mechanical terms.  (This is what makes quantum gravity such a puzzle.  The best constraint we have only constrains the correspondence limit.)  
A: The macroscopic Beckenstain-Hawking entropy formula
$$
S_{BH} = \frac{k A}{4 l_p^2}
$$
with the Planck length given by
$$
l_p = \sqrt{\frac{G\hbar}{c^3}}
$$
gives a hint that quantum gravity is needed to determine the entropy because it contains both, the gravity constant $G$ and Plancks constant $\hbar$.
However, this formula does NOT say what the correct quantum gravity is, that is needed to correctly describe the microstates of the black hole. Assuming a certain quantum gravity and calculating the entropy from a statistical mechanics point of view by counting the microstates
$$
S = -k \sum\limits_i P_i \ln P_i
$$ 
where $P_i$ is the probability that the system is in the microstate $i$, the Beckenstein-Hawking formula must be reproducable.
If it does not, the quantum gravity applied is wrong.
In summary, the Beckenstein-Hawking formula is not a quantum gravity theory, but it can be used as a test of all wannabe quantum gravities.
A: The Bekenstein-Hawking formula is obtained in the so-called "black hole thermodynamics", which is based in pseudo formal analogies with real thermodynamics. Even if we accept the formula as if was correct, it does not establish "the relationship between quantum mechanics and gravity" because it precisely ignores quantum gravity effects and treats the black hole in a classical or 'semi-classical' fashion. When quantum gravity corrections are included, the event horizon (a purely classical concept) disappears. An introduction to the kind of quantum gravity corrections expected is given in Small, dark, and heavy: But is it a black hole?
