So getting back to the subject...
I agree on the points on content, Anubis. I used to think that landscapes just sucked, but over time I've come to realize it's just that a picture simply can't see all that your eyes do... nor can it capture your feelings, the smell of the air, the feeling of the breeze on your face, the sound of the birds in the trees, etc.
There's a
lot missing in a picture that makes the scene seem wonderous to you, but boring as snot on a 2d 3x5 print.

Therefore, you have to find ways to either bring those emotions and feelings into the shot, or rely on things that can make an otherwise unexciting 3x5 interesting.
Things like depth objects in the shot that lend depth... paths carrying the eye from one picture to another... really interesting lighting (usually dawn/dusk)... textures and patterns... etc.
I think what fails most for people is just "pictures of some fields and some trees". If there is nothing in your shot other than nature doing nature's thing, it won't tend to be interesting. You have to find a compelling subject to make the focus of the picture and have the nature as the background. Nature, by itself, tends to be kinda dull.
These are just my observations and rules of thumb. They do not work all the time, nor are they necessarily "right". Just something more to work on.
BTW, also ... usually when people get into a funk about their pictures sucking... I frequently find this means that the person has gotten to a point where their eye has gotten better than their ability to capture what they see. It's a good thing. If you can bear slogging through it, when you're done, your skills will have caught up to your eye and you will be beyond a plateau of skills and capabilities and ready for the next one.