God warns him to “shun to taste,” as if Adam knows how bad it is to distinguish between good and evil. Then again, God knows about Adam's "thirst" for knowledge. See 8:8. The word "shun" comes from the Middle English word "shunnen," from Old English "scunian," which means to abhor. Perhaps God (or Milton) is suggesting hatred for knowledge? (word origin courtesy of Dictionary.com)
It's also interesting that Milton uses the word "taste" rather than "eat" here. God's rule seems to be so strict that it's not just that Adam can't eat a whole piece of fruit; Adam can't even sample it.