Environmental design is not my main area of focus but I am aware of some basics, and will try to share some constructive feedback. Also, let me know if I am wrong somewhere, I'm willing to learn!
The numbers below will refer to images in the order in which they are shown in your post.
1. That "round wall" structure seems to be man-made (well dwarf-made to be specific) and somewhat resembles a camp rather than a permanent settlement, which kinda contradicts the idea of these houses - I rather see some tents. Anyway, these buildings look way too small even for dwarfs, but take a look at these rams - they're almost of size of this two-floor house! I would either scale down rams or scale up houses, in order to make it all fit together nicely.
Assuming this is some sort of camp, how these rocks have been brought here? Probably via a mine cart, maybe leave one or two of these carts to illustrate what was happening before - small details like that make players think what is it, and when they look at the environment, they connect the dots which in effect give a positive impression and allows for better immersion. Perhaps I would reduce the number of these rock formations and put more barricades? Look up some other Hive models like this one
Wooden Barricade in case of barricades, I would put some cut-down pieces of wood (possibly near cut-down trees), indicating there were some woodwork going on to create these barricades.
If it was meant to be natural I would make it look like it so, like adding some level of height. I mean rocks don't just grow out of flat surfaces, do they?
It all comes to believability and a bit of realism (even though it's still a fantasy). You can use that rule for the rest of your map with great success.
2. Terrain height ain't too bad but it all looks too black-and-white, I'd say.
Let me elaborate - you used basically two textures there, I would add some "Grassy Snow" mixed with "Rought Dirt" on these roads (I assume those icy paths used to be/are roads for transportation?). I'd put icy textures on some of these cliffs instead, it's more likely they established there than on a road that was attended by many supposedly.
Regarding the trees - less is more; I would put fewer trees and make them BIGGER also add some noticeable variation in height. If you're worried players might get in between them you can put invisible pathblocks that are available in WE.
Wherever there are trolls, gnolls or what have you I'd also indicate that this area has been used or is inhabited, meaning instead of Snow texture I'd, again, add some Grassy Snow and Rought Dirt and possibly some props like camp fire or a hut (remember to scale them properly to fit together nicely).
3. I can see some hills and nicely spread trees in the center, looks nice to the eye on a first look.
I already mentioned roads in the previous point, but I will also pinpoint that this darker texture line looks un-natural.
4. These troggs really make themselves stand out with their bright skin. But that's just an inconsistency of a (probably) ported model from WoW with WC3 textures. In the grand scale some nice ups and downs, so I can imagine when a player goes through the level he can feel these terrain changes with the default camera.
Regarding troggs again, they look too peaceful just chilling in the snow. I know in the starting zone of Coldridge Valley they were presented like this, but for sure your creativity is way more than copying how they were portrayed in certain locations in WoW; read about their culture/behaviours
Trogg you could pick some great ideas to integrate into your map.
5. Even though you lowered the terrain where the troggs are it's hard to see that on a first glance, I'd add/change some texture to indicate these steep slopes and rock props perhaps.
A piece of the terrain on the right-hand side of a screenshot looks organic and that's the way to go.
6. Yeah, this looks badass. Some big corpse of a dragon and trolls lurking around it with some treasure, simple and on point for me.
7. Again, level of height. Dun Morogh is a mountainous area with high slopes and sometimes deep valleys; this one is too flat and quadratic (you want organic shapes when presenting nature).
Did you gather any references and take notes of some features before you began the process? Research is part of the process and is as important as messing around in World Editor.
I personally love Dun Morogh from WoW, maybe some fan/concept art pictures. And some stuff I mentioned from previous points regarding the habitat of these creeps.
8. Gnomeregan entrance? Looks majestic. But it's too empty around it, does nobody ever go there from time to time?
9. This is the best one yet, with lots of diversity that at the same time fits well together.
