Time Commitment - I think there are a lot of us who get into our own heads with this one. People naturally avoid things that cause stress and committing to run a couple times a week or signing up for a race can cause stress. To avoid this stress, they don't follow through with the training. Everyone gets touchy when it comes to "me" time. Running is often perceived as something that is going to take away our precious "me" time. What most find, is that running is a great way to energize your day, allowing you to more fully invest yourself in your pursuits. Unless you are trying to break some records, just start out small. Run 1 day a week if that's all you can commit to, it's better than no days a week.
The Hopeless Folks - Some people are so pessimistic about their own health, that they can't avoid but spreading that negativity to others. "Oh, 3 miles, I could never..." and "You ran 31 miles?! I don't even drive my car that far!" Fact is, you can and you could if you would tell yourself a different story about it. "3 miles, I could work up to that." Don't put yourself down, you have the ability to get into shape to do anything, seriously, stop telling yourself no, and start giving yourself excuses why you can.
Running Is Bad For You - Most of these folks have some personal story or have a random fact from some study that they didn't read but heard about on the News. "It's bad for your knees." This is true, if you have crappy form or try too much too fast. Running is a discipline, you need to watch some youtube videos on good running form, like this one ---> http://youtu.be/zSIDRHUWlVo. Just because you are born with legs, doesn't mean you know how to use them... Don't run when things have some sharp pains, take a few days to rest. "It's bad for your heart." This is true, if you are running a marathon every weekend at a race pace... don't be that guy. Start slow, and gain some knowledge about how to run well, that seems to be the key to running for a lifetime.