After making four completely undrinkable batches of beer, I've finally found the cause of my problem: my ****ing aluminum pot.
In each batch there was an incredibly powerful metallic aftertaste, and in some cases it was overpowering in the nose as well. Horrific stuff, I never finished a single bottle of any of these batches.
Now over the course of these last few batches I had eliminated all possible other variables, I had changed from using tap water to bottled water, I had stopped using my cooler mash tun for fear that the metal pieces in it were corroding and leaching metallic flavors, and I have cleaned and cleaned and cleaned my aluminum pot(one of two pots I use.) Each batch was using different grains, different hops and different yeasts. At the end the only variable that was left was the aluminum pot that I was still using. I didn't use it for my most recent batch, and the beer has turned out wonderfully.
So I just wanted to post this to let people know that aluminum pots can leach metallic flavors if in poor condition, and these flavors can be strong enough to completely ruin your beer. My advice is to either avoid aluminum or make sure you really take care of it well. I personally won't be using any more aluminum in my brewing process.
Anyone else with a similar experience?