Hopefully we're not hijacking this thread too far.
The difference lies in the fact that the professional world is built off of common courtesies, expectations, and standards that don't exist between your buddies. You don't care if your friend slouches, won't look you in the eyes, has a weak handshake, is late, or any other cue that employers look at. So what if you're a minute and thirty seconds late to your friend's house? If you're a minute and thirty seconds late to a job interview, you're likely be told "Don't call us, we'll call you." Even if you're not the kind of guy that upholds the standard acceptable practices for job interviewing on a daily basis, the fact that you care for that one day that it really matters means something. If you want to be taken seriously, take the job seriously.