Looks like the D100 manuals aren't available to download from Nikon's site any longer. But I did find this that should help:
http://web.mit.edu/6.163/www/pdf/D100Guide.pdf
Regarding the blue lines, those are called halos. I've seen it from over-sharpening in post processing, using maximum zoom & severely cropping a small subject in the frame. It's not something that is specific to Nikon. The Cooper's Hawk is an example of over-sharpening and using maximum zoom.
Until you use two comparable cameras/lenses from Nikon and Canon, you can't say with any certainty that these problems are all inherent in Nikon. I have used both and most issues are the result of what settings you use on the camera, not the camera itself.
If you're shooting outside football games, you shouldn't need a 4.5 f-stop unless it's a night game. The more light you have, the higher the f-stop generally. And the more depth of field you want (more focus in the entire frame instead of your one subject) the higher it should be also.
If you are shooting in low light situations, you really should be using a tripod. If the shutter is staying open longer and you're holding it in your hand, you will have blurry shots.
I use the "crappy" Tamron lens you describe (70-300 w/macro for $130.) and don't have these same issues. While some lenses do have a manual or auto override on the lens, this is generally set from the camera, not the lens.
I have shot my niece's cheerleading competitions which are indoors and are always a photographic challenge. I get there as early as possible to get the best seat, flash set on "rear" and redeye, always use the tripod, manual focus, manual f-stop and shutter and I STILL have to do a lot of post-processing. Mostly due to the white balance and the affect of those orangey indoor lights. It's just crappy conditions but there are ways around it.
Mirlyn has great suggestions, you really need to use the camera in more situations and play with all the settings to get a feel for what you need. Even with the manual, the photographic forums I post on, the photography magazines, and taking about 300-400 photos per week (sometimes more), I'm still learning and experimenting all the time. But I would never go back to film; it gets pretty expensive to shoot the same subject 20 times with different f-stops, shutter speeds, white balance, ISO...
You're kidding, right? 3 to 5 megapixels are rather common. I'd suggest going with any of the Canon Powershot models.
