A few comments and answers (some are Army specific but are probably similar in other services):
Benefits: Good question
Residencies:
For the best answers regarding civilian residencies and if you can defer to do one get in touch with the Human Resource Command for that service. They should be able to tell you if it is a possibility.
Deployments:
Army: 12-15 months (6 if you are extremely lucky) probably only once during your initial commitment unless you join a Special Forces unit.
Air Force: I think 4-6 months every 2-4 years, not sure on the frequency
Navy: I'm not sure.
Daily responsibilities:
The Army owns you 24/7. If you are with a field unit or getting ready to deploy there will be more training time beyond regular clinic hours.
You will take call for emergencies usually one week at a time, most of the time it isn't too bad. Call rotates through the docs at large posts you will have it less frequently but will get called more, at small posts you'll have it more often but it's usually quieter.
Your main job is seeing patients. You will probably have some additional duties, like infection control officer, radiation control officer, public health officer, etc. For the most part these are little admin things that you can take care of between patients. There are other projects, like putting together local dental conferences, trips to base schools, etc you might be tasked with. When your commander asks you to do something you don't really have a choice about doing it or not. Some jobs, like Division Dental Surgeon, will have more admin time assembling reports on soldiers dental status on a monthly basis which takes away from your clinic time, but you are still getting paid and just block off time to do those duties. Typical work hours run from about 7-4.
Penalty Clause: GOOD QUESTION
Be sure to ask what happens if you decide part way through school if dentistry isn't for you, or have trouble with your grades or your wife decides she doesn't want you in the service?
Base Choices:
Again you are better off talking to someone at Human Resources than the recruiter about this.
References:
Ask them to put you in touch with a few dentists and Human Resources for those questions above. Try to talk to dentists in both clinic assignments and field units if you can.
Evaluation Processes:
For the Army, there were no evaluations during school. Once you are in you gets an Officer Evaluation Report (OER) about once a year. If you do your job and complete the tasks they assign you, you won't have any problems. If you want a good one, volunteer to do a couple extra things. These go a long way in applying for military residencies.
Development/ training:
The Army will pay for you to go to a 5 day CE course once a year. Here's the catch though, as a captain, you have to go to one of their courses. The speakers are usually not bad, but they are run like the Army with classes all day from 7-5, so it isn't really a break like regular conferences. You also get 5 days permissive temporary duty, this means you get 5 days off to go to a conference of your choice without using your leave days. They won't pay for anything but you get the days off for free and you can add leave to it if you want. There are also other opportunities like the Armed Forces Institute of Pathology's week long Forensic Course that you may be able to do.
Family/spouse:
Expect 5 day work weeks with early mornings, time on call when you have to stick around town (usually need to be at the base within 1 hour if called), occasional military training time when you may be gone for a few days, occasionally being asked to stay late if a unit is deploying and needs exams or work done, lots of studying and research if you are in a residency.
Most important, the impact of a deployment on your family.
Other questions to ask:
Definitely ask that one about penalties if you change your mind about dental school or the service.
What is your entire commitment period? I believe your commitment is 12 years total, 4 for school, 4 for active duty and 4 in the Independent Ready Reserve. The IRR time is time you still can be called up, to my knowledge no dentists have been called from the IRR yet, IRR deployments are 90 days and most commonly would be used to backfill spot at a clinic that dentists have deployed from. Keep in mind that if you are in a solo private practice this could cause you significant hardship, I plan on associating when I get out until I'm done with that IRR time.
How long will the Officer's Basic Course be and where? For the Army it was 10 weeks in San Antonio when I did it. If you did it during a summer during school or before you attended dental school, it was only 6 weeks though (well worth doing).
That's really about all I can think of right now, you covered a lot of good stuff.