If you're shooting in RAW then I'd do most of the saturation/contrast stuff in your RAW program, before going to PhotoShop or whatever editing program you use. I use RAWShooter, and just play with the sliders until I think it looks alright - not really rocket science, which is good, 'cause I don't think I'd be much good at rocket science!
Anyway, generally, I like! Some specific comments, if I may...
1. Needs more saturation, and a bit more contrast, but mainly a bit more saturation. Not that it's bad as it is, but there's so much colour in that shot - bringing it out would make it. The other thing is the composition - there's not a great deal going on in the left of the photo, whereas there's a couple in the middle, and a tree to the right. I'd have been more tempted to put the tree on the right third, and the couple on the left third, ans since you look like you're trying to make the most of the foreground, maybe put the couple on the upper third. You'd probably have had to move a little to the left to do this, but I think it may have improved it.
2. Is very well exposed - you have details on the tree and some detail in the sky, which isn't easy to get. The main problem with this shot, for me, is the distracting background - was there anything behind you so you could have shot this from the other side of the tree, and not got the castle in the background? Also, could you have zoomed out a little more, and got the base of the tree? I am strangely a fan of random photos of trees, but I think you need to be able to see the whole thing, generally.
3. Is a photo that just goes to show you don't have to use the rule of thirds to get a nice photo - rules are there to be broken! If you reduce the highlight contrast a bit you might get a bit more detail in the flowers, but that's really nitpicking! Nice depth of focus.
4. As I said, I like photos of trees, and this one is of the whole tree - again a plus! The only thing is again, the background, which is quite distracting and here is quite similar to the subject you want to bring out. A narrower depth of focus might have helped, but it may just be the location wouldn't allow it. Still, increasing the saturation a bit would,
imo, improve the photo.
5. Is nice, but feels a bit lacking in detail. I'm not sure whether this is because the whole flower is not in focus, which, if possible, might have helped (the background could have still stayed well out of focus). Again, playing around with saturation and contrast will probably help.
Sorry if that seems hyper-critical, but I do like them, just spotted things I noticed! The rule of thirds is generally something I find helpful, but the thing I have felt has helped almost more than anything else I have learned is learning how to post process, at least converting and editing RAW files properly, and getting used to RAWShooter.
Hope some of that helps!
Alex