The changes to your photographs are quite subtle (with the obvious exception of stripping the color out of the black and white photograph).
The black and white photograph is great because you have very strong tonal range. You have bright whites in the lights of the motorcycle light and dark blacks in the hidden corners of the structure of the bike. This really gives the image a dramatic edged to it and makes it appealing to look at.
What I also like about the image is the subtle vignetting around the edges of the photograph. It adds an older feel to the photograph.
Your second image of the flowers is also very strong. You have a clear color palette of purple and green which again, is a very dramatic color combination. You have strong saturation levels and a sense of depth with an identifiable middle ground and a slightly blurred background.
What I would like to see you experiment more with is incorporating stronger foregrounds in your object. You did this in a previous assignment in a supermarket. Your foreground played a significant role in that photograph. Most photographs just have 1 layer (usually a background), some have 2 layers (generally a middle-ground and a background… such as your flower example above), and a few photographers try to incorporate 3 layers (with an identifiable background, middle ground and foreground). Playing with depth is an important thing to do as a photographer and I would like to see a little more of it from you. What you’ve done isn’t wrong, it’s just important to experiment with object placement and perspective.