The biggest vulnerability of a large hospital would be lack of security.  Strangers come and go everyday.  Staff would not be suspicious of someone they don't know walking into the hospital.  If you've ever been in a larger hospital, it's not hard to enter.  One simply walks in the door.  While there is security in the building, visitors are not typically screened at the entrance.  Hospitals do have lock down procedures when there is a problem, however this would hardly be effective against a terrorist.  A larger hospital would likely have more security around specific areas such as dangerous chemicals, pharmacy areas, and medications.  However, a terrorist could easily enter with a destructive device already on their person.

I would wholeheartedly agree with number three and four above.  Another thing to consider would be the possibility of using/ harvesting hazardous chemicals, potentially harmful biological elements, or radioactive materials to be employed or weaponized for use in an attack in the hospital itself or elsewhere.  The possibilities are really scary when you think about it.

Personally, I don't know why anyone would want to target a hospital in the first place!  Have people become so murderous in their hearts that they'd want to hurt already "hurt" people?  Medicine is designed to help all people and American medicine has helped many peoples (some of them Muslim).  And, to hide out in one of these facilities puts many people at risk!  I guess I'm too humane, too soft-hearted!  I'm one of those people who'd be treating all the wounded on a battlefield whether friend or foe. 

I guess a hospital would be a very vulnerable place, and certainly terrorists could make quite a statement by targeting or holing up in one.  I think it would be very hard to make our hospitals "tamper-proof." 

Having worked in a small rural hospital for a number of years, post #2 highlighted the major threat - the sheer number of people coming in and out of the facility every day. There are a multitude of reasons why someone may need to be in such a facility; a "big hospital" would have numerous entry points to facilitate accessibility by persons with varying needs and mobilities; the nature of the variety of services provided dictates that there are lots of different types of areas within a hospital that are isolated from each other (laboratories, patient rooms, operating theaters, radiology, the cafeteria, waiting rooms, emergency department, chapel, administrative offices, housekeeping, laundry). Hospitals could provide lots of places to infiltrate and to hide if someone was looking for them.

Another major threat is simply that a hospital is a major psychological target.  People would feel much worse about an attack on a hospital than about one on, say, a bank or an insurance company.  Hospitals are supposed to be havens for the vulnerable and an attack on one would be a greater blow to people's psyches than an attack on some other sort of target.

Certainly the specifics would depend on what...