I haven't been into the critique section for a while, but I'll throw in my .02. Keep in mind that these are my subjective thoughts and I have no real formal photography training, just what I've picked up flailing along (I don't know what we covered in H.S. photography class, not much apparently).
#1 - Does nothing for me. The light might be part of the problem, somehow, the highlights seem kinda -flat- to me, I' don't really know how to describe it, but there's no real glow to the colors, and some of the highlights look blown, or almost so. I also feel the picture also lacks compositionally, I don't usually like square, flat shots of natural subjects, again, it lacks depth. I might have tried an angled view of the flowers with a shallow to medium depth of field so the flowers in the background blur progressively.
#2 - For me shooting flowers, I feel like the pistol/stamen core of the flower are like the eyes of a person in a portrait - they should always be in sharp focus (unless they are oof deliberately for creative effect). This is hard with a shallow DOF, but in this shot the focus is well behind these parts, it's on the base of the top petal, and some of the top right petal. As you say, f/2.8 is a tiny, tiny slice of focused area when you're working at these close distances. I usually find I need between f/5.6 f/11 to get enough of the flower in focus. If you have a tripod, I would do this. Compose a shot you like with your camera on a tripod. Focus carefully (take a sample shot and look at the results on your display to check the focus point), then without moving the camera or focus, shoot a series from wide open aperture all the way through the smallest aperture of the lens in full stop increments. If you do this often you'll start to get a feel for what settings work out best to achieve you vision, and you can go right to those settings. I often bracket aperture a few stops both sides of what I think is the right settings are, and sometimes I save a shot this way.
#3 - The two flowers are pretty far apart, neither of them is in focus, and there are many distractions around the frame.
#4 - A classic tight flower macro, but the the DOF is too narrow.
#5 - I like this shot, I'd like it more with just a touch more DOF.
#6 - see #5
#7 - Focus!
#8 - more DOF please.
At f/2.8 and close focusing it's VERY difficult to focus hand held. Tripod or more DOF will help. Try to have a vision of what you want where in the frame and what you want focused and what the DOF should be and how you're gonna achieve those goals before you even look through the viewfinder.
Don't hesitate to kneel or lie down to get some more interesting compositions. This often lets you get a perspective which looks more fresh to people. Please don't be offended at my comentary, remember I don't really know what I'm talking about, and I still take photos that look like those of yours I griped about - I just don't post 'em.