Well there wouldn't actually be any motion blur on the mountains. That's the main reason he still looks like he's in front of a picture and not in an environment. There would be more blur near the front of the image (bottom) and none near the back (top). You could create this effect by creating a gradient, turning it into a selection/mask and then running the motion blur through it. If you want to try that and need a little more info on how to do it let me (or someone else) know.
There should be some shadow between his butt and the speeder. That's largely why he still appears to be pasted on top of it and not actually riding it.
What 4abnormal has done with the shadow and jetblast/water are definitely more like what it should look like. Using a 'drop shadow' makes it look like he's in front of a picture because the drop shadow is behind him. It won't matter how far down you drop it. The farther you drop it, the further 'in front' of the image he will appear. You want the shadow to appear 'beneath' him.
Your wake stops dead after a couple feet behind the speeder. Whether it's water or smoke, it would still have to carry on beyond the frame of the picture for us to believe he is traveling very fast.