Check out the sharpness test for 24-105L below:
http://lenstests.com/reviews/canon-ef-24-105mm-f4l-is-usm-page-2
Assuming you are shooting at 105mm alot, the images are going to be softer because the lens simply performs that way. I suggest you find out what range you use alot and get a prime lens for that focal length. Prime lens are sharper and better in just about every way except no zoom. Canon
IS performs well, I usually leave it on in all situation EXCEPT single axis panning (horizontal or vertical). Some lenses have Mode 1 and Mode 2 for static shots and moving shots, which are more useful for stabilizing panning/moving shots. Overall 24-105L is a starter/basic "L" lens, its unreasonable to expect too much. There are also sample variations, check other user's photo with same setup and see if your lens is performing to spec.
Having seen your photos, they are indeed a bit soft, but I find canon rendering softer than nikon out of the box anyway. Tweak your camera settings to see if you can do better with regard to sharpness. Additionally, you should only use top quality UV filter like B+W or HOYA HD if you must have one to protect your lens. Mid-range and cheap UV will severely degrade image quality and cause reflections/ghostings, etc. UV filters were made for a time when films were sensitive to UV spectrum, our digital bodies do not need to use UV filter as it is "immune" to UV by design.
[Edited 2014-08-08 22:50:28]