Well. . . . all kinds of stuff. Did you notice how you were only getting light colored cars and no dark cars? That's because you did a morphological closing (dilation then erosion) which expands the white bars. And other things.
What I'd do is to see if you can get a totally empty parking lot image and then subtract them. If you can't then I'd find the asphalt by converting to hsv color space and looking for pixels that have low saturation and low value - basically find out the color of the asphalt. Then I'd get the color difference between all pixels in the image and the gray asphalt color. You can do this in RGB or LAB color space. Things with a high color difference are either cars or grass. But you can do size filtering because you know that no car will ever be as big as a lawn so use bwareafilt() to remove grass. Grass has high saturation and a hue in the green region and a size larger than the known size of a parking space. So with the grass and asphalt gone, now all you have is cars. But there is still the problem of dark cars because they are close to the color of dark asphalt. There again you'll have to use size information - cars are going to be no larger than the area of a parking spot so you can get rid of large dark asphalt areas. You could also look at the Solidity (returned by regionprops). With cars it will be close to 1 and for weird-shaped asphalt blobs it won't be close to 1.
