It is covered with hills of netherrack and lakes of blazing pyrotheum. It is adorned with spheres of iron (no steel block dimlet, unfortunately), ringed with tendrils of gold, and it menaces with pyramids of firestone ore. It is dotted with small lakes of blood (though I'm not sure if there's any visible in these screenshots- they tend to spawn underneath the hills).
Also note that the spheres aren't solid. Inside, they're filled with black oil.
Also, well-timed flying pigman. I bey he got kicked by a dragon. All the usual Nether mobs (save blazes) spawn here; the pigmen are just the most common by far. But there's ghasts and magma cubes, too. The ghasts don't tend to last long- they get run over by dragons, and then they die. That's actually where the gunpowder on my hotbar there came from- a dragon killed a ghast somewhere over my head, and the gunpowder literally fell into my inventory.
As it turns out, Buildcraft oil is highly flammable. It was surprisingly difficult to get the dragons to tear into the orbs in such a way that the oil would spill out onto the pyrotheum and catch fire. I ended up having to float next to and kind of underneath the left-hand orb here to lure a dragon where it would make the oil spill out.
I didn't cause the spillage on the left side of that orb, though, or either of the ones on the right orb. I blame that bored wither.
Dragons, you say? Dragons. There's four in this picture; I'm not sure if I ever saw five at once or not. So their population cap is four or five. I want to say McJty said the cap was four dragons and five withers when he implemented their dimlets, but I'm not sure.
Also, note the sky. And the red fog down the valley, which I think was a nice touch.
They definitely damage the terrain. There are no caves in this dimension- this did not exist when I generated the dimension.
What is this I don't even...
Judging by the fact that there's more than twice as many portals here than I ever saw dragons (at once, at least), and the number of dragons flying around did not decrease noticeably, I'm pretty sure they respawn.
Also, you can see the corner of one of the blood lakes here. That slightly brighter red rectangle on the edge of the portal just above that dragon's right wing... is blood.
Finally, punching the dragons spawned in this dimension with syringes works just like punching the one in the End. So a dimension like this is a renewable source of dragon blood, for all your dragon-blood-requiring needs. I bet a dimension like this would horribly break Draconic Evolution as well (like it's not pretty broken already).
Those withers spawn naturally, too. They sometimes fly up and shoot at the dragons, but mostly they float on the ground and shoot explosive skulls at random. All those little patches of flowing pyrotheum that you see around in the middle of the lakes are from bored withers.
So infinite nether stars AND dragon hearts. If only some of the terrain could be made of draconium ore...