Please add more.

- Don't make the layers 3, 4, & 5 out of the same stuff. It confuses the player. At least make them different shades, if not different types of tiles.
- Mask things intuitively. If it looks like you should be able to step on it, make it masked. If it looks like you should be able to fall through it, don't mask it.
- Use brighter colors and more detail in the foreground and sprite layers. The further back you go, the dimmer the colors and less important the details should be.
- Be careful what you attract the player's attention to. Like the previous point, if it's not something the player is supposed to look at, don't make it flashy or obvious. If they are supposed to look at it, then make it flashy.
- Include stuff! The more stuff your tileset can do, the more likely people will choose it for their levels.
- Take your time. It's way better to have a slow release of a good tileset than short releases of crappy ones.

...what else?