Heh, my wife and I were there just recently. As to the shots...
#1 The horizon line distracts from the impact of the sea oats against the sky. Such a strong line tends to divide an image making it difficult to see the whole rather than two separate halves. I would suggest considering shooting from a low vantage point to show the entire upper part of the oats against the sky with the horizon at the bottom or perhaps even not show at all. I would also consider getting closer to one in particular to see what you think. I love the idea, and the top part works well, but I think that it could be stronger still. A tripod won't help sharpness here because these guys sway in the slightest breeze, but using a higher iso would help, and since you want a soft background shooting wide open might give sharper results too. Consider taking multiple exposures so that you have at least one in focus.
#2 That's a fantastic area, rich in wild flowers and uncommon beach plants, but I don't get a lot of that feel from this particular view. I do like the leading line of the boardwalk, but it doesn't lead anywhere visually interesting. Again, I think that getting in closer have more impact, perhaps showing part of the boardwalk (it has great texture) and some of the plant life. Then again, if you went when the sky was interesting, like when there was a storm on the horizon, the leading line would be very useful.
#3 Birds in flight are often interesting, but this one is cast in shadow and the background is not overly visually appealing. This has great separation of the feathers in the extended wings, but I feel like I want to see more details in the bird itself.
I think that in this instance you would have had better results shooting at a time of day when it would have been better lit. Spot metering off the bird would just have let the background blow out (be very overexposed), although the bird would have had more detail. You might try working with these guys in mid-day when there is more even lighting. They stay out from dawn to dusk so if you are patient you should be able to get some in-flight shots again in the future.
#4 This one is really strong in my opinion. Great detail, sharp eyes, close crop. These guys are common here, but I've not seen many in the grass. I like the lighting myself, it gives him a bit of a glow around the edges, and great texture in his carapace. They can get aggressive at times, so beware your toes! Nice work with this guy.
Overall these are all fun to look at and not at all bad, so please don't read too much into my comments. That location is a great spot to photograph, although it gets popular later in the morning and down right crowded in the afternoon.
You mentioned grids in your response above. These are, at least in my experience, most commonly found in alternate ground glass focusing screens, which may or may not be available for your camera. The easiest thing to do might be to simply pull a rule down into the image area in photoshop and then rotate the image so that the horizon line is level. Alternately, you can rotate the image, scroll it up so that the horizon meets the top of the window you are working in, and then rotate more to get it level...
- Randy