1) Location
Depending on your location, you might be miserable or happy for your four years of medical school. If you are one of those people that can just pick up and go somewhere and be perfectly happy no matter where you are, then location probably isn't a big deal.
However, if you are picky about living in suburbs, or city, or in the middle of nowhere, or in an area of the country that is really hot vs really cold, etc., this may impact you heavily. Despite what people generalize about medical school, you WILL have at least a LITTLE time each week to yourself, and people tend to underestimate the value of this time spent - it is necessary to keep yourself sane. Therefore, if you are in a location that doesn't provide some semblance of life that you desire, you may find yourself going crazy by the end of the first semester.
Also, depending on your location, you may have family or friends nearby. A support base is very helpful in medical school, and may even be of paramount importance if you have children that need taking care of. Location may be important if you have a relationship or spouse, you may want to be located near them, and also some medical schools are not located near airports or other points of transportation in case you do long distance.
2) Prestige
This is debateable - I have been told at interviews during the orientation talk portion that prestige is still factored into residency admissions, that residency directors around the country still have an idea which schools are known to be competitive. Generally speaking however, the level of prestige of a medical school for residency may be somewhat equivalent to the level of prestige of an undergrad institution for applying to medical school. It may be factored in, but it will be minor compared to other factors like grades and standardized exams.
3) Cost
This is probably one of the biggies - we all know medical school costs are rising, and the time it takes to pay back loans is no walk in the park. Thanks to George Bush, the interest rate sucks, and causes an influx of money threads in the preallo subforums here at SDN.
In state tuition varies some, in certain cases like CA or TX, the difference is pretty large. But you must weigh what you are actually paying for, and some would claim that for schools like those in the top 10 like Harvard and JHU, it is worth it. Others claim that paying debt back for up to 20 years is not favorable to their long term goals of buying a house, having a family, etc., so they opt for state schools that generally have a much lower tuition cost.
Look at what you want out of life down the road, and what you want in your medical school education. Weigh those criterion with the money, and find the deal that fits you best.
4) Likelihood of being accepted
This is an important thing to look at when applying, if you are a 3.0/31 GPA/MCAT student, there's really no sense in applying to the top 10 and nothing else. The general scheme used is pick a few reach schools, pick alot that are more within your reach, and pick a few backups. This is an expensive and tedious process, you want to maximize your chances of acceptance because going through this process again is not beneficial to your application or your wallet.