Maybe blah, but you've already hit on one of the issues, no main subject. You like the horse and the green slats and the muted brown on the wall.
Although the green slats may be interesting you probably don't want them competing against the horse for attention. Photograph one or the other and make it the main subject.
On that note, get in closer (or zoom in closer) to fill the frame with your subject. More than half of the image has nothing to do with the subject(s).
(negative space can work in some cases, but it doesn't here -- probably at least in part because your subject is hidden behind a wall of negative space already.)
The image is not sharp.
The image is not completely in focus. Most likely your shutter speed is too slow and the horse moved slightly.
Finally, the subject is not very compelling (The horse also looks ill or half-asleep) and flat.
I don't believe this photo is worth a great deal of effort in saving (sorry you said have at it), mainly because it is a bit out of focus.
But improvements can start by;
- crop it such that the right wall is gone
- on the other side, put the horse's eye on about the top-left-most 1/3 intersection lines. A bit more breathing-room on the left is actually better and is a good reason to break "the rule of thirds" here, so extend the crop a bit further to the left.
- Increase contrast, and clarity.
- although you like many things in the scene, you may also want to crop severely and just get small portions of the image, Just a part of the horse's face/head. Or part of where it's mane meets its neck, etc... Or just the deteriorated wall. Or just the green slats. Any of these can make a compelling image on their own as well....
Just some suggestions. None will make this an award winner, but I think there is room for improvement on this image. Or at least some things to keep in mind for the next one.
Hopefully not too harsh.
i.e. Going to Flickr for a "good job" on every photo, helps no one.