Emerald Hill Zone:
This level is not hard to edit. Mostly tags are unnecessary for doing any work on it, but there are a few. The loop and spinning platform are most definately tags, though, so they are a little hard to make. Otherwise, the level is pretty easy to edit. Just try to make sure all of the tiles fit together like a jigsaw, because it will be glitchy if they don't connect properly. The main trick in this level is being patient enough to find the tile you're looking for, and not using too many underground areas, as the underground tiles aren't very varied.
When editing EHZ's palette, you shouldn't have any trouble either. Just watch out for the tile with the waterfall over the platform, as the yellow pixels in there are the same color as the rings, so when you change that color, the rings will also be changed.
Chemical Plant Zone:
This level is a mixed bag. The metal platforms are so easy to work with it's child play, but there are plenty of irritating things here. First of all, the blue tube-like platforms are weird, and some of them are not solid. Secondly, the loops have quite a few tags associated with them, which makes it very close to impossible to move them. Lastly, the warp tubes are downright impossible to deal with. I have NO idea what the format is, so the only warp tube I have in Sonic 2Z is already there, and I moved the others into locations you can't reach =P
Palette editing is a bit hard, as the background buildings have the same color as the rings as well. If you want to do this level well, you'll need to edit the tiles a bit.
Aquatic Ruin Zone:
This zone is plain EVIL. I have no way to describe how annoying editing this level's tiles are. It's not that any of them are weird and don't do what they look like, but making a level here is like doing a 5000 piece puzzle with 3000 pieces missing. The tiles are VERY hard to connect and you WILL have to change plans many times. I really don't know how to describe how hard this level is to edit. There are also plenty of tags running around, many of which I have NO idea the purpose of. This level is pure chaos in all senses of the word. When editing this level, remember that the only consistant tiles in the whole level are D8 and D9, which provide a wall blocking the player. Also, using the 74 XX trick mentioned in General is a good idea to prevent total insanity.
Fortunately, the palette isn't hard to do at all, and that is at least a breath of fresh air.
Casino Night Zone:
This level is decently difficult to work with. The tiles aren't very evil, but the objects are quite annoying sometimes. The objects used to allow for the pinball machines will drive you insane, especially when trying to make a really sweet area only to find there are isolated glitches covering it. When working with this level, you need to pay attention to every tag and object, as any rouge object you don't know what is could cost you some time. Also, note that the large red bumpers in the tileset are not solid, and don't do anything. If you remove them, they aren't really removed either, and they'll bounce the player despite not being visible. I have no idea where these are stored, and it's quite amusing to see as Sonic is being bounced around by invisible forces =P
As for the palette, you shouldn't have too much trouble working this one to your own ends, just remember that the format for any section that changes color isn't known yet, so anything that flashes can't be changed.
Hill Top Zone:
This zone is, in a word, impossible. The tiles don't appear inside the game as they do in the level editor, so this level is basically impossible to work with. The objects appear to be easy to do, but I refuse to deal with such an insane level to work with. Maybe later utilities will fix this problem for level editors and make this more sane.
The palette is easy, except for the lava colors.
Mystic Caves Zone:
This level is incredibly easy to edit. The only challenge is finding the correct tiles to connect. The objects are easy to work with, and the tiles almost all do what they look like. Also, unlike the other levels, most of the ceilings stop the player from moving up, which is incredibly helpful when making stages with multiple paths.
This level has a very simple to edit palette as well.