Looks great, looks cold. Makes me want to return to a place I love.
Composition-wise it doesn't do much for me, but for anyone who has ever spent time there, it certainly tugs on the heart strings.
If you ever saw a house that had a single straight roof line parallel to the horizon, you will understand my slight uneasiness with the composition. I guess I want to see some framing, or some flowing slopes...something to break the straight horizontal layers.
BUT! I like it just the way it is...a very nice image.
That looks like you had a fun time in Yellowstone. I like the shot, my only suggestion would be if the the 3 Bison were more to the right in the frame rather then dead center. I think as a composition it always works best when animals are entering the frame rather then like the first bison that is already on his way to exit the frame.
I really like the way that the trees are leaning in the same direction that the bison are walking. And for each foreground bison, there seems to be a corresponding tall tree. Perhaps it would have been even better with a less centered composition, but there is a lot to like here as it is.