Non-Mormons are NOT Going to Hell. In Fact, It's the Opposite.
Most other religions believe that we will be with God in heaven. So do Mormons. And not just us--everyone will be with God after we die.
Other religions believe in a "trinity" god of Father, Son, and Holy Ghost. Although we (differently) believe they are one in purpose and not in person, LDS Doctrine nonetheless teaches that everyone will commune with God in heaven. In the "lowest" kingdom (which we call "Telestial"), inhabitants experience the companionship of the Holy Ghost. In the middle kingdom ("Terrestrial"), they can commune with Christ, and in the "highest" ("Celestial") they can be with the Father. So, in this sense, according to LDS doctrine, everyone who expects to live with God still gets what they're expecting.
The main difference in the Celestial kingdom, then, is NOT whether we escape a place called "hell" in order to get there. There are many heavens, all of which are glorious, and essentially everyone goes to one of them. The main difference in LDS teaching from other Christian teachings, then, is not whether we are with God in heaven (because we will be, even according to the traditional Christian sense), but whether we want to become Gods ourselves. That requires a large measure of
(a) responsibility on our part, and
(b) trust on God's part, for Him to share everything he has with us.
Furthermore, there is no shame upon someone who does not want to become a God. God still loves each one of us just the same--unconditionally--and he will grant to every one of us, regardless of which heaven we settle in, something far more glorious than we can currently imagine.
So, when Mormons say that not everyone will make it to the Celestial Kingdom, we're not saying that "ya'll suck" or that somehow "everyone but us is going to hell". We're ACTUALLY saying, "take what you believe about heaven, and add to it your wildest most pleasant dreams, and THAT's what it can become for you if you want it to."