-Well about the trees, a couple are from a different tileset from the rest - they have a more greener tint to the bark then the others. Its not a huge complaint but they look a little like they don't fit in.
-Well you can still keep your symmetry, just make it look a bit less perfect. Its not symmetry thats the problem (though it is part of it if taken in a certain context), its just how everything looks so uniform. I can tell you used the plateu tool with the rectangular selection. Maybe just make the top level a little less even, maybe go against your symmetry desire and make it a bit more rounded. You don't have to completely wipe the terrain, just work to make it look a little more natural.
-The waterfalls need some work. Use more and space them a bit further apart yet still overlapping (Basically use more of them and vary their spacing more). Also I would suggest using some of the raise and lower tools to make the area around the waterfall conform more - water carves into the land its flowing through so the edges around it would be smooth. To make the stream look a bit more smooth. Go under the advanced tab in the editor and uncheck "enforce water height limits" - this will let you lower the terrain right next to water next to a cliff without it forcing the "fence in" effect around the water so it can flow through the cliff and not over it.
-I'd also suggest more random environmental doodads in the first environment, other then the focal point the terrain is pretty devoid of natural things like shrubs and plants (I personally like the one shrub doodad that doesn't generate pathing (so it doesn't interfere with movement) and placing a bunch of them together to create tall grass).
-You should also work on the border area's around the maps, you fade to black when you get to the map boundaries when a real forest/desert would extend far beyond this point. If it helps go to view and uncheck shadows so it removes that boundary fog making it easier to see and visualize that area.
-About symmetry, its fine on a large scale but remember that things don't normally grow or form perfectly symmetrical in nature and thus you need to remember this when terraining (unless your doing like an advanced city where symmetry is good)
-Out of all the terrains, I have to say that I like the frozen forest the best (though the random green grassy cliff in the corner of the screen looks incredibly out of place, must be a super grass able to survive weather like that).
Its a good start, it just needs some more work.
About doodad limits, if your doing like a solo RPG then you could always break the map down into more detailed smaller ones to extend this requirement (if your aiming for an ORPG this option sadly isn't open unless you make like several "Chapters" maps of it, each a detailed quest into a particular zone. Hey you could even make level requirements and the such for each "zone")
I think your on the right track, especially with that last one, and for the other areas try adding some wave to the earth like some small rolling hills.
