It is all relative. On one hand adcoms want to know that you are capable of handling a busy or difficult schedule, but on the other hand they also realize that undergrad and med school are two different animals. If you work or are busy with lots of extracurricular obligations, then it is perfectly acceptable and will not hurt you at all to take slightly lighter course loads, as long as you do well in them. You won't be working or be involved to many ECs in med school, so it's not fair to ask you to handle a med students course load while in undergrad.
For example, I am married and have to work about 30 hours a week between a part-time job at a hospital and another 10 hours each week in a research lab on campus. I also enjoy going to school year-round and taking the 2 sets of block classes during the summers (8 week semesters instead of 16 weeks). So I usually take 12-14 credits/semester instead of the more traditional 15-17 and then take two summer blocks for another 12-15 credits. I actually earn more credit in a year that way than someone who overloads and takes 18 credits each semester but doesn't do any summer school, and my load is pretty consistent through the whole year.
So the point is to find what works for you based on your circumstances. It is far better to take slightly lighter but balanced loads and perform well than to try and impress an admission's officer by trying to bite off more than you can chew.
