It's easy to forget composition when shooting flowers. I usually do this by paying close attention to what's in the background - and what type of contrast the background will have with the flower - both color and value contrast.
Another thing (besides it being centered) is that there is no stem. You do not always need to show the stem, but with no leaves and nothing anchoring it visually, this one looks a bit like it is just floating out there. Just my opinion, of course - as you know photography is very subjective - what I like may not be what you do!
If you use a shallow depth of focus, as you've done, focus on an emphasis point, something that you want the viewer to see specifically. Often times the emphasis point is the center of the flower. I've found that if the center is blurred, the whole shot looks bad. It's like taking a portrait with the eyes out of focus. With this rose, I would say there isn't a point that is really dramatically special...an opening bud might be more interesting to shoot. As people have said, try shooting from the side, or include other parts of the rose in the frame.
One of the best ways to improve composition is to look at lots of photos and figure out what you like about them.