I cannot give an exhaustive answer for everything you could improve, but I think you have a problem I am frequently noticing in my own images: oversaturation. This is especially well visible in the third image - in my experience, dead reed does not have such a rich golden hue, especially on a cloudy winter day.
Oversaturation happens frequently, because a) some tools are succeptible for it (darkroom + enhanced colour matrix on Nikon D90 images, but probably other cameras too), b) it is the side effect of some widely used techniques (changing your curve to an S-shape is usually recommended for correcting contrast, but it also increases saturation) and c) when you compare two images, which only differ in saturation, your impulsive preference is almost always for the more saturated one. (In case you ever wondered why TVs and monitors have such hideous colours when exhibited for sale, it is because traders turn up the saturation to max to exploit this cognitive defect).
Result: you run your image through your raw program, and it looks better. You run it through your image editor, change the curves, etc. Then take your result, compare it to the input, and like it more. But if open your own image after a week - or somebody else sees it - it looks completely unnatural. If you don't spot it outright, this may be, because your examples here are mild when compared to some of the transgressions I created before started paying attention.
Cure: First, learn to look critically at your own pictures, explicitly paying attention to saturation. Some subjects can bear big deviation in saturation without losing their natural look. If an old house is made of reddish-brown stone, who is to know that the stone was more brown than red in reality? And sky often looks better with more saturation, because it often looks pale on pictures exposed for objects on the ground. On the other hand, there are some subjects which are sensitive to saturation differences. One of the best examples is human skin (or at least pink Caucasian skin, I don't have experience with other races). Oversaturate it, and the person will get a flushed red face with yellow blotches.
If you decide that you want less saturation, just use the saturation slider (after you have applied other saturation-changing edits like curves). Here you have a problem: how much? Remember, with every decrease of saturation, your brain is telling it that the picture got more boring. So I find it best to use an objective tool. There is such a thing as a saturation histogram and when you oversaturate, it gets clipped on the right side, just like the RGB histogram gets clipped with overexposure. Get an editor which has this histogram (I find a linear version better than the logarithmic one, YMMV), and move the slider until it isn't clipped left or right. You cannot stretch it, so you probably want to put it as far to the right as possible without clipping, leaving the hole on the left. This is your natural look. Then think whether you want your picture to look like that or if you want to add some oversaturation for the image to be able to grab looks.
Other things I notice beside oversaturation: An S-curve is generally suggested for images, because it emphasizes midtones and creates greater contrast, which is good for your typical gaudy image where a subject was exposed as 18%. This may work well in the second image, and the third definitely benefits from the increased contrast, but it is a bad decision for the first. There you already have very good contrast between subject and background, no need to emphasize it further. But you are suffering from loss of details in both shadows and highlights.The shadows (the sheep's noses, and the wool of the black sheep) are not especially interesting in this image. So I'd just leave the shadows on their own, risking to drown them, and concentrate on bringing out more detail in the highlights (snow, which is very expansive here) at the expense of the midtones (sheep, which already exhibit lots of structural detail). I'd also up local contrast after that, so the wool doesn't get too flat (will help with the snow details too). If you have an equalizer, you can increase the middle-to-high frequencies too, but I don't think it is found in most editors.
This is a crude try on the first picture. The curve I used turned out to be quite strange, but it worked - you see the trampled snow behind the herd and the nuances in the virgin snow before them. I applied local contrast, but the equalizer crashed :( I also (sloppily) removed the distracting holes in the foreground, the sliver of puddle in the upper right corner and a suspicious dark spot from behind the herd (or do I spend too much time around male teenagers?). I didn't think to remove the left most half sheep by myself, but it turned out really good in che's version, so I shamelessly copied his idea.
At the end, I'd say I got the snow better, che got the sheep better coloured, but somewhat dark for my taste, and your picture is not very natural, but well suited for advertising, or somewhere where cartoonish exaggeration is a plus. Pick your preferred look, or a combination of them, and make the picture the way you want it.
Well, you hooked me, I spent the evening playing with your pictures. BTW, it turned out to be a nice exercise - editing pictures when I don't know the intent with which they were taken.
So here is your sheep close-up. Not much changed, except they are now wool coloured, not rosy-peachy. (If your editor has options like vibrance, velvia, or natural skin tones, turn them off in the sheep pictures, they are meant for portraits)
And now my rendition of the lake. I am the first to agree that it is far from a natural look. But I loved the pattern in the water, and when I righted the trees and stretched the perspective to align the lake edge with the horizon, it got a much more prominent place in the composition. So I increased the contrast, smoothed the surface and fumbled with the tint until it had a dramatic steel look.
I don't expect everyone to agree with that last proposal, it is quite radical. If you want to make this image natural, just reduce the saturation in what you've already done and cool the colour temperature back to what the camera measured, then you'll have a nice conservative edit.
Edit: a more detailed explanation on how I did the sheep.
White balance: I set it to 5200 or 5300 K, I don't remember exactly which. If you think that makes it too gloomy, leave it at 5000 K. The warm look you have used is strange for an overcast winter sky.
Exposure: I turned it up as high as possible without clipping any channel, not just the combined RGB.
Curves: Here is where magic happens. This is a very close approximation of my curve.
There are two things to be said here. First, I custom-tailored the curve for the image, so please don't take this as advice how curves should look in general. Curves are one of the most important tools in photography, and if you want to be good, you have to understand them for both fine-tuning the S-curve for "standard" images and custom building a curve for difficult images like this one. I linked one of the best learning sources I know of in a comment below, read the part about curves (and if you don't know it, the part about histograms too). Second, the bump on the left is a "kids don't try this at home" kind of hack, you probably want to make the curve smoother there.
Exposure again. In theory, this shouldn't happen, but the Curves change was way too radical and made the image too dark. This time, I did it by image look, not by histogram, monitoring darkness vs. loss of snow detail and stopping at a point I could live with.
Local contrast (optional). This is not the same as Contrast. Not found in every editor. If yours has it, you can try it and look how it changes wool and trampled snow.
(very optional) I think I applied a very slight graduated neutral density fake on the upper 30%, but don't remember for sure. Worth trying out for more detail, but can make it too gloomy.
Composition. I removed some distracting elements (in GIMP, not in the raw editor). If you don't know how, there are lots of tutorials on the web. It is probably best to choose one where the process is screencasted in a video. And while you are at removing, take away not just the big things, but also look out for some stray snowflakes on your lens which manifest as almost transparent grey spots.
That's about it. I did not change saturation, but by the look of your images, it can be that your editor applies some saturation-related enhancements by default. If your sheep are orange instead of a muted beige, reduce the saturation using the slider.