I know all too well the perks and problems of rendering, as many of you may know. I started out thinking of all the awsome things I could make and I wanted to show them to everybody. The result of doing that gave me essential feedback needed to improve my work over the years. The one thing that is always a constant is the need for good lighting.
You can have a great scene with great models and textures, however, when it comes down to it, it's the lighting that will determine if your image looks great or not. If your scene depicts a light source (star, lightning, artificial light) which would fall upon your models, then you need to generate the lighting to support that. This is really true when rendering against a background instead of a full scene.
If your chosen background image has a light source, then you need to position your lighting to match it. This way, the light in the image look natural and not fake.
Now, for some friendly criticism: I notice that the majority of your images seem to be a little off in their lighting. Some of your renders have a huge light source shown, yet no light from it is hitting your models. This can cause disbelief. I also noticed that there are times that objects in the background image are lit by an off scene source, however, your model is not lit from the same direction.
These are just a few things to keep an eye out for when creating images that require specific lighting. Here's an example of what I mean:
First Image: Notice how the light is not coming from the bright background? a dead give-away of poor lighting.
Second image: I positioned my light almost on top of the bright spot in the image. If you can view your background in a viewport, it makes doing this easier.
Background image is from Deviantart.com
