Hi, I just joined this site cause I want to learn more about matte painting. I took a matte painting class at my school and really enjoyed it, I did this for my final but would like to learn how to improve it and make it ready for projections.
Looks pretty good overall. My two pieces of feedback are:
The bright foliage in the far background may well be part of the reference you are using, but I think it doesn't really work as part of the overall image. I'd expect it to be more muted and misted out, like the distant foliage on the left of the image.
The black levels in the car don't match the blacks around it - have a look at the very darkest shadows on the tree roots near the car, and then the shadows on the car. They would be very similar in reality, but here the blacks in the car are much darker.
I think there are also some (slight) issues with angles of your reflections as well, and it would be great to see some kind of disturbance, variation or detail in the water - like algae floating on the surface, or the stream bed below it. At the moment it's a perfect mirror sheen. Again, this is possible in reality, but looks a bit odd even in real life...
Hope that helps - would love to see more. Would be great to see your starting plate, too.