I've never been to the Marshall islands, but have had friends who have worked there.
It seems to me that you have 2 questions: 1) what is it like to work in the Marshall Islands, and 2) what do I need to know about being an expat?
1. There is a school of thinking that island countries have a certain mentality. The work ethic is less, deadlines take on new meanings, and the Spanish "manana" looks desirable by comparison. In island countries, their whole world is the island (although a goodly percent of their population may be offshore, sending money back). There is a sense of insularism that many find very frustrating. Dependency on foreign aid may be high, and may contribute to a sense of lethargy. Being a small country, it's likely that everyone is related to everyone, so nepotism, cronyism and internal networks are strong (so transparency and merit-based employment may be lower).
2. As for being an expat, you want to make sure that your salary is competetive, paid in an exchangable currency, and not subject to local tax laws. Depending on your employer, you'll want to check out whether you are US tax free (i.e., if you have taxes withheld or not; if so, you are eligible for up to a certain amount of pay to be tax free.)
You'll want to get your head around the cost of living, to make sure that you can live (and save) on your salary. There is likely to be higher prices for a lot of things, since much of it will be imported. You may want to check out airlines connections and fares, to know what your likelihood of getting out is, and the extent to which you might be able to get others to visit you. You'll want to know basic things about the quality of life (schools, safety, housing, availability of items) and you'll want to know if you'll have language dififculties.
A good website to check is http://www.talesmag.com/rprweb/the_rprs/pacific/marshall_islands.shtml (broken link). Wow. Having written that, they have no entry for the Marsall islands! That's a first for that web site, which has non-official information provided by mostly State Department people. You could search for entries for nearby countries, and extrapolate the findings.
Alternatively, you could try to go to the source.
Marshall Islands (01/08) is the State Department's page (good news, there is a US ambassador). If you can get a copy of the Post Report (an internal, nonsensitive State Department document), that will tell you a lot about what you want to know in terms of quality of life. If you can't get it straight from State, ask your COngressman to help you. Post Reports are written for incoming Embassy staff, and for those who are up for rotation, to help people identify where they want to go next.
Good luck with it!