I don't buy anything with MSG in it, it plays havoc with my seizure disorder. For the same reason, nothing with artificial sweetners. My neurologist has confirmed that both artificial sweetners and MSG can be major neurological triggers for people who suffer from migraines or seizures.
I also try to avoid things with HFCS. I don't think its a catastrophe if we eat a little of it, but I do my best to avoid it.
DH loves Mountain Dew, takes one a day with his lunch to work. He REALLY likes the Throwbacks (cane sugars, not HFCS), so we are stocking up on them while we can get them. I was SO excited to read that they will be releasing a "Heritage Dr. Pepper".. I don't drink hardly any soda at all, but sometimes I crave a Dr. Pepper SOOOO badly. Now I can have one with no HFCS!!
Only other things I refuse to buy are onions. Everyone in the house hates them - DH and I literally feel ill if we eat them.