I won't lie: I see a LOT of rookie mistakes. However, I want to be constructive and to help you think better about what you want and how to get it, rather than just blasting away. Because you are still taking a class, I'll limit my comments to those items that are easy to fix, i.e. that you could apply in an hour or less to really improve the image.
(1) Align your objects. Half of the objects (for example, the coins) are resting *above* the table surface, and the other half are resting *below* the surface of the tablecloth. And what's going on with the cabinet-doors behind the counter? Objects that don't look like they are lying naturally will catch the audience's eye immediately.
(2) Don't use low-resolution textures, like in the front of the counter, the back walls, and the floor. You'd be better off using a built-in noise texture that doesn't capture a feeling of stone, than having those clearly pixelated images so prominent in the image.
(3) Use lower-saturation colors. What that means is make it a little more gray. If you have full control over the numbers for RGB (like 0 0 255 or maybe 0.0 0.0 1.0), then you can make a color more gray by adjusting each of the three numbers so that they are closer to the average. For example, 255 0 0 is super-bright red of a laser, while 64 64 192 is the duller red of a Star Trek uniform. I understand you want vibrant colors, but if *everything* has vibrant colors, it doesn't look colorful so much as contrived. The walls, arches, scrolls, and that yellow-and-wood chest in the center should be less noticeable than some of the most brightly colored potion bottles. Also, don't underestimate the value of using a "noisy" texture (in Blender 3D, one of them is called "Clouds" which I use a lot), and make it randomly mix two very closely related colors. The subtle variation will look like real-world imperfections and lend depth to all of the materials.
(4) The lighting is all wrong. At the back wall, there are shadows on the yellow arches. Where is that light source coming from? It's confusing. Also, the light coming from the upper-left (I think?) is too bright: tone it down a bit. Do you have some sort of ambient-light turned on? Because I think that there isn't enough contrast between light and shadow, particularly given how deep the shadows are in the right-side foreground. If that area is dark, why is the front of the counter so light? Lighting is definitely one of the hardest things to do right in a 3D model, but I think the image would be much more effective if you only had one or two dim lights in the whole thing, creating a huge contrast between shadow and light. Then again, that may be a personal preference. Also, using area-lights instead of point-source lights can give everything a "soft" look, more like the last image you posted of these geometries with no materials.
The geometric models are still pretty good, though. I mean, I can totally see some polygons, but you can't fix that without redoing the object. But the geometries are not the weak point: it's the materials and lighting. Those elements can make or break the image, and it'll take a lot of practice. Maybe once your class is over, come back to this and play with lighting and texture modifications for a while. You've got a great foundation: now you need to hone the color-space details.
Wow, this is more insight than I ever expected from Newgrounds (lol). Yeah, I really rushed to texture this in time for midterm. I really wanted to do a lot more with the lighting, like imply some candlelight and possible edit in some fire effects post-render. What I used in the previous image was 3DS Max's Ambient Occlusion Pass with no other lights than that.
I spent too much time free-balling as I went with the model and as I mentioned before I didn't consider many things when starting the project (like the glass). I know that this is a learning experience and hopefully I'll be a little better with my final still life project.
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