1) Would have loved to have a narrower depth of field on this one. However, your EXIF shows you used f/1.8 at 50mm on a crop body, so you have in effect a mild telephoto, with some compression of background/foreground going on. In this situation, perhaps a wider lens (24 or 28mm), and closer focusing distance would have separated the foreground from the background a bit more.
2) Like the overall feel of this, but the OOF brushes on the left side tend to diminish the impact of the image for me, and the bit of detail at the lower left is distracting. In this context, opening your aperture to the widest value may have helped keep the DOF narrow and focused our attention on the brushes in the mid-plane.
3) Nice. But I’m wishing you didn’t cut off the tube on the right side. I like how you have the focus sharp on the lower right and using DOF to have it blur to the upper left.
4) You cropped too tight. Obviously having a car vertically is unusual, but we’re missing the context. So the photographic choices here include: is the background part of the story or not? If not, then use DOF and a closer angle to blur it and to separate the object of interest from it. Secondly, here again a wider angle lens would benefit, in that you can get closer to the image, have it fill much of the frame, and at the same time use the wide angle lens’s ability to create distance between the foreground and the background.
Note: I do realize that all these were taken with the 50mm lens, and if that’s what you’ve got to work with, then please disregard the suggestions to use a wider angle lens. However, within the context of using a single lens, you’ve got to use distance to subject and aperture with its control over DOF to either pull together foreground and background, or deliberately separate them.