Generally speaking, is a fusiform abdominal aortic aneurysm considered to be a potentially serious problem? If so, in what way? Can it cause severe pain even if it doesn't "burst"?

Potentially serious. An aneurysm's risk is related to leak or rupture. The chance of that happening is related to the size (diameter) of the largest cross section. The size is monitored and intervention is usually not suggested until it's >5cm. The other possible reason to intervene would be rapid growth of the aneurysm. In the WORST case, a free bleed from a rupture, you can bleed out all your blood into the abdomen.
AAA. Ansolutely it can potentially be a life threatening disease. It can cause severe abdominal or back pain even if it doesn't rupture.