To summarize what others said before and add a few missing points:
What edge tiles should not have:
walkmeshes (no problem if there are some, but why add them if they are ignored anyway?)
tile lights (these HAVE to be removed because they can cause crashes)
source lights (never tried if they cause crashes, just don't use them on edge tiles)
shadows (might cause crashes, not sure, but standard edge tiles don't have shadows)
animeshes
animations (these will cause crashes, it's important to remove them)
nearly forgot the most important one, emitters (these are a sure way to crash the game)
What edge tiles can have:
tilefade
textures with procedural animations like water, lava, etc (but not linked to an animation node)
What I never tried on edge tiles:
other lights than main and source lights
Bannor said that you can't use "Textures that are animated such as tree leaves". Depending on what he means by this he could be wrong. If he means you can't have leaves linked to an animation node so their transparency works better, he's right, this doesn't work for edge tiles, but if he actually means that the textures have txi files so they'll be animated, like water, etc, he's wrong. You can have water, lava and so on on edge tiles, just don't link it to an animation node.