You are no longer weighted down by your class, alignment and other concept that have been choking the creativity out of RPG titles. This game's selling points are S.P.E.C.I.A.L. system and intelligent script.
S.P.E.C.I.A.L. character development system basically lets you be anything you can be. There are no classes (thus no class restrictions), the only limiting factor is how many points you have placed in that skill. And considering the sheer amount of options during point distribution you can play the game several times and have a unique experience each time around.
The other ace up Fallout 2's sleeve is interaction with the world that is intelligent, humorous and extremely interesting. If you focus your points on sweet talking you can cross almost the whole game without firing a single bullet.
Fallout 2 takes the brilliant concept of Fallout and takes it to the next level. Pretty much everything from Fallout 1 ius here, but on larger scale. New improvements are drivable vehicle for quick transport across the wastes and enhanced control over the NPCs in your team.
It will make you laugh. it will make you cry. It is everything a role playing game should be.