I will answer some questions, which are not necessarily what you asked, but what you want to know.
What makes terrain "Blizzard-like"?
Blizzard terrain has two set of rules that are never broken: it never uses custom imports, it never customizes any objects with Object Editor.
Is that a good thing?
No, that is not necessarily a good thing, as it limits what you can do drastically.
What are typical "features" of Blizzard-made terrain?
Blizzard often combines water and cliffs in quite unnatural ways and has very weird doodad placement. This makes it look unnatural and unrealistic, but it does make it look like Blizzard terrain. Rocks in odd places, waterfall descending into a small pond, a bunch of rocks jammed together in exactly certain count, two or three rocks placed near one another at the edge of a forest. These are examples of repetition that can be found in Blizzard terrain if you look for them. Another very common thing that can be seen in Blizzard terrain is greatly exaggerated usage of raise tool, especially under trees and forests, and lack of its usage in watery areas and flat areas.
Common false claims:
Blizzard Cliffs are bad and ugly - this is completely false, they are just fine and most functional option, you just need to know how to use them.
You can create good-looking terrain only with custom imports - this again, is completely false, you can make very good looking terrain with the set of tools given you by Blizzard.
Smooth-looking terrain is good-looking terrain - this is as false as a claim can get. Said in many tutorials, this leads to very bland and way too feather-edged usage of tiles and doodads. Contrast is what gives detail and attention to a scenery. Too much or too little, they both look bad. Using less is easier, than using more, with caution. However, using more with caution gives you better results than using less contrast.
Many doodads and decorative objects give you good looking terrain - false. Terrain with few objects can look good. It all comes down to what the place is supposed to look like.
Playable terrain cannot be good-looking - once again, false claim. Surely playable terrain won't reach the detail level of purely aesthetic terrain that can be seen in a single screenshot at a time, but it can still look good.
What can I do, to make good-looking Blizzard-like terrain?
Do things better than Blizzard did. Keep things they did well, and improve in areas they did not do so well in. Decide what you want to keep and what you don't want to keep. Decide what individual decisions you make and make the terrain follow that set of rules from start to finish.
What areas Blizzard terrain sucks at then? What can I improve?
Blizzard sucks at using raise/lower tool. Almost any terrain looks a lot better with a little raise/lower tool (exceptions are city floors that SHOULD be completely flat). Never forget to use raise/lower tool. Use it both, before and after placing objects on the ground.
Blizzard sucks at setting object height. Blizzard almost never sets custom height to any objects by holding down [Control] and pressing [Page Up/Page Down] -keys. This can be seen as self-levitating rocks, trees and other objects all around Blizzard terrain. They can be considered as a feature of Blizzard terrain, but if you ask me, it is just laziness, and you can make a lot better - just as Blizzard-looking terrain by improving in this area.
Blizzard sucks at rotating, tilting and rolling objects. Once again, a feature that Blizzard never uses. You should use all tools given to you - this includes rolling and tilting different objects, just slightly to increase the amount of different variation in your terrain - slightly angled trees and rocks increase the immersion of a single scene immensely.
Blizzard sucks at placing objects naturally. Blizzard has tendency to follow a set of guide-lines to not do certain things, like using tile-variation beneath forests, or place one kind of flowers next to each other. Such things would be completely natural and logical and it looks good. There are things you might want to consider before doing, as they can affect gameplay - like placing rocks inside a forest? Does it affect gameplay too much? Personally, I don't think so.
Blizzard is dead-end terrible at polishing terrains. Blizzard never polishes any terrain they made. This can be seen as inconsistent decoration and object placement. Illogical transitions between areas and lack of different objects used in their terrains. One could take a screenshot of Blizzard terrain and then try to "remaster" it and see how it looks. Personally I can say that I could do a lot better than they did. Thanks to this list above.
What is Blizzard good at?
Blizzard is good at making empty areas. Empty areas is something that SHOULD exist in terrains. Often you see every place decorated to top-notch and with highest detail possible. But this is not always good. There are empty areas out there in real-life too. Blizzard too, leaves certain areas empty. Why? Because it increases contrast compared to rest of the terrain. One could say: less is more.
Blizzard is good at tile-usage. Blizzard knows how to use their tiles (most of the time), not always. They blend tiles very well with good contrast. However, the time they used to make terrain was not very much, and thus too big palettes were used most of the times, resulting in poor tile-variation when you zoom in enough.
What restrictions you can break? What you cannot break to create Blizzard-like terrain?
Custom Imports you CAN import custom-resources as long as they are Blizzard-like. Usually this means using Blizzard-only textures and similar polycount as Blizzard used. Avoid using custom-made units. Skins not allowed. I'd recommend keeping it to minimum, only if something really helps and would look good and blend in well.
Manipulating Objects this is something you SHOULD do. Make minimum size of objects as low as 0.01 and maximum size as big as 6, even 10 for some objects. Rotate them, tilt them, pitch them. Scale the height and width separately, turn off collision and do it artificially.
Set Custom Height this is something you SHOULD do. Set height for trees and rocks, especially structures.
Use height variation (almost) everywhere those small bumps, lowering and raising the ground makes it look alive. Do it. Do it better than Blizzard did. Exaggerate when it is needed, otherwise use small variation.
Use all brush sizes don't stick to the big ones, nor the small ones. Use every and each one of them. For me, I use brush sizes 3, 2 and 1 the most, in that order. Only place objects with brush size 1, with exception of trees, that you can place with brush size 2. When making terrain (height and tile variation) use many brush sizes, going between them back and forth and back and forth again, changing the direction and tile and brush size used.
Create new objects don't stick with the limited set of objects given to you. If something Blizzard-made might look good, try it out. If it fits. It sits. Color things if they don't fit, try to make them fit. Use your imagination.
Pile Objects this is setting custom height version 2.0. Place objects on top of one another. Why would there not be a rock on top of a bigger rock? Use that height setting.
Don't make objects clip Blizzard almost never does this. They don't do it because they don't make any adjustments or changes to the objects used, and pathing size prevents this. But you can do it better by adjusting everything and making sure nothing clips - this is something many beginners and even adept terrainers forget. Don't place grass inside a stone - it does not belong there. Don't place a tree on-top of stone, they don't grow there.
Back to the thread, descending from the clouds (of writing):
I mostly do my terrain with Blizzard-only resources and I've done so for years, so I could say I have the experience, I'd recommend checking my album. A few examples:
