What camera did you use?
I really like #1 and #6 for composition, but you've suffered from the same problem we always struggle with - the curse of the landscape photographer, if you will: blown highlights.
The landscapes are great, I love the lighting in #4, but the highlights let you down in several of them.
There are a few ways you can avoid them next time:
1. Shoot in RAW (if your camera allows it):
This will give you a better dynamic range and a bit more to play with in photoshop.
2. Underexpose, until there are no blown highlights:
Your camera may have a highlights warning built in (overexposed parts of an image will flash when you view them back). I use this all the time. You could also use the histogram
to see if there are blown highlights. This will rescue the highlights detail, and there will still be detail in the shadows, which you can bring out by tone mapping in Photomatix or a similar HDR (high dynamic range) program, or by dodging and burning in photoshop.
3. Shoot multiple exposures of the same scene and then combine them in an HDR program, and then finish it in photoshop if you want to.
4. Use a graduated ND (neutral density) filter which is darker in the top half, and can be used to darken skies, but this works better with straight or nearly straight horizons.
For anything to do with HDR or photoshop, you should probably ask our resident expert, Alex, for help.
All in all, the photos are good, and if you don't want to spend time (or money) doing those things and you're happy with your photos as they are, then don't.
Good luck! I'm looking forward to seeing more from you.