One tip for you about why it may not feel right is that it still has a cartoon feeling. A few things you can do (if this is not what you're going for) is:
1) Use thinner to no outlines. Line weight is crucial for sketching but thick outlines in renderings make the image look cartoon-ish.
2) Add some texture. A boot is perfect for being able to add heavy duty fabrics and materials that have distinguishable textures.
3) Work on shading. You have a glow/highlight on the near side of the boot, but you also have a shadow cast on this same side...thats backwards (or I'm not reading it right). If its a reflection instead of a highlight make that clear so this isn't confusing (certain materials may have a reflection of the ground like hard plastics/rubbers but not as much for soft rubbers/fabrics. The shadow that is cast is also overlapping your horizon line...this really hurts the effect but is a simple fix!
Good work, now take it to the next level!