My parents have only met one of my boyfriends--that's because I live almost 2,000 miles away. They liked him as my boyfriend, but not as anything more. I didn't understand that then, but as the relationship failed to progress, I saw what they're saying. The funny thing is that I have friends in similar relationships, and I want to tell them to run for the hills! But that's a decision we all have to make for ourselves.
Sometimes, parents have a hard time with someone because of their religion/race/social class, and those complaints are effed up. Equally messy are the cases where your parents don't think anyone is "good enough," or they don't want to share you with anyone. But, if your parents don't like your significant other--and this gets important as you approach marriage--because of substance abuse, perceived possessiveness/abusive tendencies, failure to hold a job/provide for themselves, or anything like that, then their concerns are justified.