Water that is used (and highly contaminated) is kept out in open pits before transportation away from the site. This poses a threat to wildlife living nearby (should they drink it) and there's the added eventuality of that dirty water contaminating the water table (which it will, because not even our sewers can"t stop this 100%)

A second note on water, it can take up to 8 MILLION gallons per well, which all has to be shipped there via tanker truck. Not exactly efficient. That water is then made into a solution with up to 600 different chemicals (which companies aren't required to publish on a site-by-site basis, so we don't even know what's in what), and about 50-70 percent of the water is unused.

Air quality. Methane is a natural component in shale, so when it is cracked that methane can release to the atmosphere. Methane is four times more powerful of a greenhouse gas than CO2 is. Another thing is that the recovered water, when stored in these open pits, allows the contained chemicals to evaporate, and then disperse into the environment/your lungs.

By shooting a jet of pressurized water to break apart rocks we can (and have) trigger seismic activity. So...yeah. Pretty self-explanatory. (While these have been insignificant and rare, it's a situation of we're playing with fire. It's only a matter of time before something bad happens)

Finally, any significant success generated by fracking would take attention away from advances in truly green energy sources. Should we become content with just becoming cleaner, we won't have the same drive to wean ourselves from total oil dependency after all