All in all, moles are really quite harmless - even when they are big, ugly, hairy and just downright unappealing. In most cases, the biggest impact that moles ever have on people lives is simply in terms of their appearance and the psychological impact that it can have on a person's self esteem, confidence, and so on. While this impact is certainly nothing to be scoffed at, the point is that the physical danger of
moles is really rather minimal - accept in one very particular case.
The one case in which moles can actually be very harmful is when they turn cancerous. Some moles have a chance of turning cancerous but the vast majority tend to never do so. It is unknown why some moles develop into cancers while others don't, but the fact remains that this is a very real threat and one that you should keep an eye out for.
The moles that may develop into melanomas (i.e. turn cancerous) are known as atypical moles. These moles are generally bigger than a pencil eraser and have an irregular shape and pigmentation. If you have any such moles on your body - or if any ever develop, then you should keep an eye on them just in case they ever actually do turn into a melanoma.
Some of the signs that you should look out for include ulcerations or bleeding, pain, itching sensations, changes in size, and so on. Just because a mole appears to be growing or bleeding does not automatically make it a melanoma however, but it does mean that you should check with your doctor just in case it actually is one.
If caught early, melanomas can be treated quite effectively.
All things said and done, the only thing that you really need to be concerned with at this point would probably be keeping track of any and all atypical moles that show up on your body. That is the first step towards catching skin cancer early (if it actually does crop up). By keeping track of the potential targets, you should be able to spot any change within them and catch this problem very early on.
If in doubt, remember that it is always better to err on the side of caution and visit your doctor anyway. Skin cancer is not something that you want to fool around with or take chances with - period!