A description of what Mystcraft is? Alright.
Mystcraft is a tool for world creation. Out of the box, with no additional adjustments, you can make many new worlds/dimensions, each with different features -- colors, suns, moons, etc. You can give them different landscapes -- the standard world you know, the end-style island, the nether-style cave world, a strange "tiled" world with 4 chunk by 4 chunk landscape-style chunks, etc.
You can control the biomes in these worlds, as well as their frequency. For example, you can make a world with 10 ocean biomes, and one plains biome -- you'll get lots of islands in a sea. You can then shrink those biomes size with small biomes or tiny biomes. One example of this was someone who used EBXL's red rock biomes, tiny biomes, lots of ocean biomes, and a red color scheme to make a very nice world. Or, what I saw earlier today, a deep pure-red color scheme for a blood magic quest (HQM) age.
But that's just out of the box. What can you do with some tweaking and mod config files?
How about a vanilla overworld, with all your mod content found in ages? Additionally, all your mod content is biome-based generation -- COG for biome-based ores, Plant Mega Pack for biome-based plants, etc. -- and now you have a major reason to go exploring.
Vanilla is boring. Sorry. It is. All plains look alike. All extreme hills look alike. Etc. The only places where things don't look alike is at the edges.
And with Tiny biomes? Not only do you break out of 1.7.10's horrible climate zones, you now have LOTS of biome edges. Things look ... interesting!. I never played beta 173, but people said it was the best terrain gen. Well, the worlds I get playing with tiny biomes look a lot like the pictures I've seen from 173. (Disclaimer: This was in the older 147/164 style biomes; 1.7.x has very different biomes definitions and behaviors.)
Now, can you do more? Sure. The lore of Myst said that the D'ni were able to make worlds that were very different. Heck, there's a different kind of lore (Amber), where we see people traveling to different worlds, where diamonds are sitting on the ground for easy harvesting. Mystcraft can do that. You can make a world with easy access to diamonds.
Doesn't mean you'll like the world. After all, a world where things form so very differently might have other differences. Air might not be pleasant, and you might be poisoned. Gravity might be heavier, and you might be slowed, or have mining fatigue. Other, generally minor things as well -- at least, minor until the world becomes seriously unstable. As the world deviates more and more from overworld-style worlds, you might see catching on fire from exposure to the sky, or perhaps a rain of meteors (explains why there's so much ore, right?), or ... worse.
At the extreme, you'll find that the world you are exploring has a death sentence, and you will be fine only as long as you keep moving; as soon as you pause, or try to set up a base, the death of the area around you will force you to move.
When the world is falling apart on you like that, you'll probably want to go home. You did bring your way home with you, right? You didn't die, and drop it on the ground, or just plain forget, right?