I've never had a complaint about the bokeh on mine, I see your images and agree that's not very appealing. However, I think the prior post is right, you my be too close to your focus point. I found it odd that the F5 shot had a better looking background than the F4 shot. Likely due to the distance from you to the focus point.
|AF4_2483 Surfing the Serengeti by DaveInHouston|
from Hot Air Balloon view
|Peregrine Falcon by Psychic1|
from Best Wildlife Photo of the Week - 4