With Rifts, the game is constantly changing because it's a dynamic quest system. The class system is very in depth, there are 4 main classes, Warrior, Cleric, Rogue and Mage, each of those classes have 8 subclasses and you can select 3 of each subclass to form your class. So, that makes for a lot of diversity in the way you play the game. As for me I play a Cleric with 2 healing specialties and a shadow damage spec so i can quest and do dungeons well. The game world is pretty large for one without an expansion (at the moment since it's in beta). The Gameplay and Graphics are very top notch and there are a large variety of crafting skills. You aren't bound to a specific leveling scheme since there is dungeon, grinding, pvp, rift closing and questing. Combat is very smooth and pvp is fairly balanced. The Quests are very interactive and there are Uber Rifts (not called that) that occur each day where some kind of demon or something attacks a main village and you need to team up with other players to take it out, you get good items and exp. Closing Rifts gives items to players that allow you to buy special gear.
I don't know what else to say...