You die.
No, seriously, peanut butter is high in omega-6 fats, so that's not good for you. It's also linoleic acid, which needs to be converted to the needed forms in order to be used, but an excess of linoleic acid will use up the enzymes needed to convert it. The thing is, the same enzymes used to convert linoleic acid are the same enzymes used to convert alpha-linoleic acid (omega-3 fat, found in flax oil). This can shut down the EFA pathways, so you better make sure you're getting EFAs from things like fish oil, eggs, animal fats, and organ meats.
It also contains phytic acid, which inhibits nutrient uptake of zinc, magnesium, iron, and some others that I can't recall off hand. Eating lots of it is not good with respect to this. It also contains enzyme inhibitors, so eating a lot wouldn't be good for digestion.
This isn't exactly a bad thing of peanut butter, but many people often replace good animal fats with peanut butter, which means you're not getting the fats that you need.