Typically if you come right out and say something depreciating you won't get a single comment because people figure "well, if he says it's meh, far be it for me to disagree with him". (just fyi)
The image has both composition and technical issues, but the technical issues are a lot easier, so le't start there.
- Your color balance is off. The entire shot is very green. This could be just you needing to adjust things differently, your monitor may need color correction with a device like a Spyder, or both. I would actually bet it's more of the former.
- The whole shot is out of focus. Neither the tree nor the background elements are focused.
- The horizon level is unclear to me. It seems to me that the camera is tilted, but it's hard to be sure since the horizon looks flat. Picnic benches aren't usually at angles like that, but I dunno...
Aside from that, the composition itself doesn't seem to be overly interesting. There IS some interest in what is going on in the tree bark there, so it's possible if the focus was better it would be intriguing, but generally I've found this kind of shot to not be that appealing. Toss a person in there posing with the tree and you likely have a winner, but the tree itself may not make the shot.
I hope this helps. Keep at it and congrats on the upcoming D300.