A couple of things that comes immediately to my mind are:
1.-The sand is too uniformly gathered at the top side of the hourglass. Im sure you were using this picture as a reference https://upload.wikimedia.org/wikipedia/commons/7/70/Wooden_hourglass_3.jpg, and you can notice that the sand at the top part makes a minor slope. When trying to do realism, subtle aspects like this one are the ones that separates CGI from Realism.
2.-Flat texture. Your main texture is fine, but it is flat regarding its color. When looking at the reference, you can see that the texture becomes darker when it comes closer to some edges. This can be replicated by 3 different methods:
-Dirt paint. It involves vertex color paint. Easy and cheap. (Several tutorials for this available at youtube)
-Ambient Occlusion baking. Just naming it for you to know it’s existance. It can be useful for other things in the future.For this project,It is not the best approach though.
-Pointiness Value Node. Actually, this imitates the way you would use Dirt Paint for this task, but it is way more faster, and real time. I googled a quick tutorial https://www.youtube.com/watch?v=SYslucCSnPI
With pointiness Value node you can imitate the way Wood becomes darker, burnt, at edges. This would increase the realism in your shader by a whole lot.
3.- Im not sure, but I think Im seeing seams on the texture. It is very noticeable at the left pilar, top part. You should fix that, because CG artists see these kinds of things really fast, compared to regular people.
4.-Fresnel input. It is something really hard to get a grasp about how to use it, and why and how it helps for 3D realism. But you definitely should take a look about this.
Some cool tutorials about this:
https://vimeo.com/103108876 .Layer weight has been the way most Blender artists have been imitating Fresnel. Some like to use the Facing input, while others, like in this video, are using the Fresnel input.
https://www.youtube.com/watch?v=pNiVB7tRG68 . CynicatPro is a Blender Artist that makes really good, informative tutorials about realistic materials. He explains fresnel in an awesome way, but he also makes difficult shaders (but shares them) in order to get more realism. Take a look at him after you finished your main, latest project. It will test your knowledge about shaders, even probably your faith (?).
Hell, Im sorry if I suddenly gave you too much information.
Last words: I would fill in a little more the blank spaces. Add some interesting shapes, like little pyramids, or cubes gathered… Add a texture to the wall.
Your lighting seems really good for me. I like the reflection. The post process with ambient particles, fuck yeah. I would love to learn how did you make it or what kind of footage you used.
PS: man I wrote too much, sorry!