You should change camera view and choose different rendering resolution (also aspect ratio 4:3 doesn’t look good for this one)
Show less grass and more background.
A lot of vertices on plants shouldn’t be a problem. I think it’s because you have the water reflection in the scene. Do you have AO turned on? It doesn’t look it is… It usually slows rendering significantly, but it looks cool in most cases.
Also why isn’t your grass casting shadows? Watch this tuturial to get better looking grass: http://www.blenderguru.com/the-secret-to-creating-realistic-grass/
You should also sculpt the rocks a little bit. Just get some noisy brush and start sculpting. It’s really quite easy to do such easy things like rocks.
I would add more rocks and plants if I were you. You can get something from cgtextures for the background, but edit the file in Gimp or PS first, so it matches your scene mood.
edit: now I see that you asked for some ideas on composition - turn the camera to be looking straight (you could then add some rock with moss and the already mentionied waterfall to get rid of background) and move it more to the ground, thus the details on the rock and flower will come out. Also, what about adding some fur on the flower?