Slimes spawn at the very bottom of the map (or they should, but I think the spawn code got bugged). They need a certain amount of room to spawn, and maybe flat ground. I'm not sure exactly, but I've noticed tons more in the mines I've dug myself than deep caves.
Hollowing out a large room seems to be the best way, but make sure its big enough. My first hollowed mine was large enough that they would spawn away from the ladder. My second was a little too small and I couldn't even go down there anymore, lol. For some reason after they spawn they gather around the corners of the room, so I get four swarms of them.
BTW I haven't seen one at all since the update, so I'm not sure if the bug got fixed or not.
As for the nether I'm a bit disappointed. It was cool at first, but now I think it needs more variety - maybe ectoplasm lakes, or soul trees, or new resources. Before the update I was liking the idea of having to get lantern materials from hell, but if its going to be so bland I think I'll change my mind.
I also think ghasts need to spawn less often, but take more than 3 arrows to kill. The interruption to building or mining lightstone is annoying, but they're not actually that tough. I want to be scared by them, not irritated.