JOEL - We don't "have" to dedicate two years of our life to a mission; we "get" to dedicate two years of our life to a mission. Serving a mission is an honor and something that most all active young LDS men look forward to with great anticipation. See the recent news article at: http://www.deseretnews.com/dn/view/0,1249,415016218,00.htm
A ward missionary is someone who is called as a misionary but stays in their home ward and continues their normal life(school, work, etc) in addition to serving as a missionary as time permits. A ward missionary is not a full time job, it is more like another calling that anyone else might get in the ward, such as a Sunday School Teacher. Full time missionaries dedicate their entire time during the two years to service, and are called to serve someplace far removed from their home ward.
As far as being denied our eternal inheritance for not serving a full time mission, only God can be the judge of that. People can serve in other ways throughout their life time without going on a 2 year mission. But it is something that all young men are encouraged and expected to do if they are physically and spiritually capable of doing it.