One of the first things that might spring to mind is: what kind of creatures will be down there? This is a valid question because the conditions an under-house crawlspace provides are often ideal for a variety of animals, rodents and insects, some of which could be poisonous or dangerous especially if they are feeling threatened. With very little room to maneuver about down there, you could find that a hasty retreat is difficult.
Molds and fungi are likely to be found also as, again, conditions are often ideal for it to go. Standing water or sewage is another dangerous hazard you could encounter.
If you think the home has possible structural issues with it, a under home crawlspace is a dangerous place to enter and it would be best to have a professional inspect the home for you. If it becomes necessary to enter a crawlspace for whatever reason, though, its very important that someone knows where you are and what you are doing. That way they can be ready to raise the alarm if you encounter any problems. Finally, before you enter a crawlspace, make sure you have the proper protective gear needed to be safe from anything you may find down there.