While total mileage is important, it doesn't tell the whole picture - 15k miles in stop and go traffic is much harder on a car than 30k highway miles.

Overall condition, maintaintenance records, oil changes, minor problems repaired, tires in good condition, paint condition - was it washed and waxed, engine compartment clean, no dents and dings, body seams straight and consistent, drives straight and quiet, shifts smoothly...These are the most important things in my opinion.

I think mileage is given more weight because it is easily measureable, but with the tolerances that engines are manufactured under today, 100k miles is easily achieved and even expected. With proper maintenance, that is.