I also am not above imprisoning them for periods of time. I’m thinking of doing something with a piece of glass and a green screen.

Reduced to 800×600 with GIMP.

Someone said the white flowers in the first pic are overexposed. True, but complicated because the stem looks well exposed, and if you lower the exposure for the flower it’s going to go dark. You can fix that in Adobe Camera Raw (ACR can handle JPG and TIF, not just RAW) by using the recovery slider to bring the white flowers down, or by shooting it exposed for the flowers and using the fill light slider to bring the stem up. Or, probably most simply, using a polarizing filter when you shoot it to cut the glare off the flower petals.

I know you said you usually shoot in RAW, and several people have told you to shoot in RAW. Would it be redundant for me to tell you to shoot in RAW? 😛
Digital cameras handle shadows better than highlights. The RAW file will contain a crazy amount of information in the shadows that you can’t see until you use something like Adobe Camera Raw to bring it out (fill light slider).
Shoot in RAW, expose for the highlights, take care of the shadows on the computer.

Depth of field, composition and exposure are all tools that can be used to create a wide variety of effects as are film grain and digital noise; the trick is to understand the tools and know how to use them to express your intent. Photographers have made great art with $20 plastic cameras and Polaroid Instamatics as well. They knew how to embrace their tools and make what they wanted to see and knew they could get.

I agree with much of what’s already been said, but not all of it. That seems reasonable because we shouldn’t all have the same vision. If we did, there’d be no more room for artists. My first thought about the red tree is that it would look very bad centered (as would most subjects), but that I might have tried a different composition. (On the other hand, I wasn’t there so I don’t really know.) It’s entirely possible that centered would have been a good choice. My second thought is that, as Dave says, horizontally centered might not be so bad, but I’d have to see it to know. On the other hand, the subject of that photo is very clear to me.

Some basic tips:
Higher ISO = more noise but a faster shutter speed or smaller aperture.
Smaller aperture = more depth of field but a slower shutter speed and more motion blur.
Faster shutter speed = less motion blur but a larger aperture and less depth of field.

Diagonal lines = usually good.
Subject at 1/3 points in the frame = usually good.
With digital, look at your histogram (funny graphy thing) and try not to have anything bumping right up against the left or right sides as this indicates loss of data in the dark or light areas. Sometimes this is impossible; learn to deal with it.
Nearly all photos can benefit from a slight curves adjustment to increase the contrast.

The cardinal rule of digital photography: Always. Shoot. RAW.
With RAW, you have more control over the processing of your image: The file has more tonal range so you can control the contrast. The file has no color adjustment so you can control the white balance. The file has no lossy compression so you can control the artifacts (you’ll still end up with a jpeg, but you’ll want to keep the file RAW for as long as possible. Once it’s a jpeg, you can’t go back.) Store the image in RAW if you can, I use Lightroom (I think ShotWell does this, but I’m not sure).

General thoughts on the actual photos:
I like daises (or whatever) in the second from the bottom photo quite a bit (it feels like the left side is ‘up’). Your sharpening the upper few is more heavy-handed than I tend to like. Birds are hard.

The ‘enhanded’ flower is a really interesting effect. Shifting the yellow to green and blowing out the white of the petals so there’s no detail really draws my attention to that one. It reminds me of an over-cooked hard-boiled egg.

If you want to learn more about photography and reading is your thing (and I know it is), there are thousands of websites out there that are dedicated to just that. Here are a couple:
http://www.bbcwildlifemagazine.com/masterclasses.asp
http://digital-photography-school.com/