A good number of the overworlds from some hacks of here are blocky, flat and boring. Here I am going to give you some tips of how to make a good looking overworld. Oh, and feel free to give some tips as well, since this is the idea of the thread.
1. Plan what you are going to make, the worlds' themes, the number of worlds, etc.
2. Before start building the OW, delete everything at the map. Everything includes the 8x8 Layer 2 tiles, 16x16 Layer 1 tiles and the events.
3. Look at the SMW overworld before making your own, too. SMW's overworld is an example of a good overworld. It isn't flat, blocky and boring, it has relief. It contains plain areas, mountains and cliffs, and it also has nice decorations. After taking a look at SMW's overworld, start making yours.
4. To don't make the overworld blocky and boring, try to add a variety of tiles in an area.
Here's an example of a blocky area: it's composed by a few different tiles and it doesn't have too much decoration.
Here's an example of a nice area: it's composed by a variety of different tiles (including corners) and it has some nice decoration.
5. Changing the palette and the graphics of your OW is also nice. Changing the palettes may make your overworld more joyful. Changing the graphics isn't necessary, but they give the idea of a totally new OW.
As you can see, there are a few graphics changed in the screenshot above, also, some palettes were changed.
Also, please, don't make bad palettes that hurt the eyes.
6. You can use some OW sprites too, as for example the Piranha Plants that can give an interesting appearance in your overworld and the Boos that you can place on Ghost Houses or haunted levels like a graveyard level.
7. Try to match the levels exactly with the OW.
Example: You're not going to put an underwater level in the middle of a dry land!
Follow these tips and you'll have an awesome overworld. Also, if in your opinion is something missing, post in this thread your own suggestions for a good OW.