Been seeing a lot of landscape photography by some very enthusiastic photographers. There's some great photography being created.
However, I do often see a few common issues that can be corrected.
Crooked Horizon Lines: Because of nature (as in bent trees, poles that are actually falling over); or the nature of wide angle lenses - a foreground element may appear to be crooked if the horizon line is level.
Don't let that sway you - the horizon is level.
There may actually be a good reason for the horizon to appear un-level such as a curved shore of water - but that's an illusion and usually means you have to re-orient your camera. Sometimes its as easy as just keeping the camera level side to side and front to back. Sometimes it means you have to move up or down to get the proper perspective.
If your horizon appears crooked - you're photo looses effectiveness. Elements in the photo can be a little crooked - but if the horizon is off the photo will never be as strong as it could be. So really pay attention to that when you look through the camera as you shoot. Its not easy to spot slight problems in level and perspective, but the more you can learn to see that in camera the less time you'll spend on the computer.
If the horizon is obscured by hillsides, mountains, etc - you may have to play with it a bit to figure out what is correct.
There are tools to put on your camera to make sure everything is level - if you continually have problems with level this can be a great investment (and yes I have one in my camera bag.)
Now - if you do miss something, that can generally be fixed in editing. Cropping and rotating can be done with every image software that comes with cameras, or you can find free programs like GIMP (open source image editor - GIMP.org) or with online programs like Picassa, or with higher-level editors like Photoshop or Photoshop Elements.
If you use Photoshop - there's something in the last few editions called "Puppet Warp" which can straighten errant objects if need be.
Centered Horizon: The second thing I often see - and honestly this is something I learned very recently - is the horizon near the centre of the image.
Now, I've known the Rule Of Thirds for a long time so I rarely put the horizon in the middle anyways. The image is typically just more interesting with the horizon one third of the way from the top or the bottom.
But there's a bigger reason - which I mentioned - I just learned. The visual sense of depth of the image is greater by using the Rule of Thirds. By placing the horizon in the middle, you loose the sense of depth.
Now creating a 'flattened image' is fine if you're creating an intentionally 'graphic' image rather than a 'pictorial' image.
Typically however, we're using foreground, middle ground and background to help the viewer sense the same depth which we experienced as we shot it. However because we're translating reality from three dimensions into two dimensions, we have to use tricks to keep that sense of the third dimension and the Rule of Thirds can help with that.
Not 'using' light to best advantage: Light can hide or reveal, it can bring forward or push backward, it can create moods or not. Many landscape photographers try to only shoot at the "magic hour" which is really about 5 to 10 minutes on a cloudless day when the sun has dipped below the horizon but light is still plentiful.
That's not the only light to use - although it is beautiful when you catch it. Any time of day can be good, but look at where the light is working to create a great picture, and where you might want to 'let that one go' or at least realize it won't be a prize winning shot. Nothing wrong with that - we photograph to capture memories of times and places and not having a picture of something you want to remember is worse than a 'so-so' picture of it. If you can arrange to come back to the same place when the light is better - perfect.
But, sometimes that just isn't possible.
The factors of light to consider: Direction, Quality and Colour.
Direction: Light from the side brings out more detail than light from behind you. Light from behind the subject can create drama.
Quality: Is it mid-day sunlight which is very harsh, or is it an overcast grey day which happens to make colours in the foreground very saturated? Is there spotty light that can create the dramatic shots (best when you're located under the cloud but an interesting feature/mountain/building is in strong light.)
There is no "bad" light - just "un-interesting" use of it - if its cloudy, concentrait on small subjects, if its sunny concentrait on the big. If the sun is behind you - either look left or right for a subject, or rotate around your subject to get more detail.
One photographer recently suggested if you're shooting mid-day, convert the photos to black and white. Most of Ansel Adams fantastic photos were taken mid day and black and white was part of why they became iconic (although he did do some wonderful colour work as well.)
I hope that helps you create some great landscapes.