Invisible, but not silent
Theoretical developments should ideally make contact with experiments at some point. The trouble with the field of quantum gravity is that deviations from known physics are expected to be tiny - so tiny that these are practically impossible to prove experimentally. Black holes provide a natural arena for testing a number of proposals, especially through processes that produce gravitational waves.
Building blocks of gravity
We are pretty sure that general relativity has to be modified at short distances (high energies), but we do not know how. This theoretical problem has a long history, and we are probably still far away from solving it. Among the plethora of models to explore, those based in the analogy with condensed matter theories provide a fresh and manageable approach to some thorny questions.
The weight of nothing
The introduction of the cosmological constant is sometimes referred to as Einstein's biggest blunder. The cosmological constant seems to be a mysterious nexus between the low-energy and high-energy physics of the universe. By this reason its study is considered as a valuable tool in order to improve our understanding of the ultraviolet properties of the gravitational interaction.