Chris,
Sounds like you had a good trip. Panama is an amazing place for seeing herps.
In general your shots are pretty good. I hope you don't mind if I am a little brutal in my remarks, but for me at least, that is how I improve.
I am going to look at your photos the way that I look at my own and give the sort of comments that I make on my own photos. Please don't take offense, I only want to give you some ideas that may help you improve. If you don't like my suggestions, feel free to ignore them. Photography is part technical and part artistic, so you have to capture what you want the way you want to.
Most of the problems aren't exposure issues but rather composition issues. Here are a few specific comments.
This first shot loses some visual appeal because of the strained look on the frog. Treefrogs with their big eyes and appealing faces are generally better photographed looking towards the subject, and in more relaxed pose. The empty space to the right of the frog could be cropped out and probably some of the tree trunk.
This cane toad is one of your best shots. I think I might have composed it without so much empty space on the right. You should make sure you don't always put your subject dead center in the frame.
I also think this shot might be improved by being a little lower. If you take pictures closer to the critter's level, you get more personality to the photo.
First of all, I am jealous of this find. I was really hoping to find some Centrolenid frogs when I was in Panama, but didn't find any.
As for the photo, I think the main issue is the position of the frog in the frame. I would like to see less of the black on the right.
Also, the key part of this image is the frog's eye. They are really amazing and you should make sure that is where the image is at its sharpest focus. It appears the image is focused on the front foot instead.
Also, with frogs with incredible eyes like this, it generally is better if they are looking more towards the camera. This other photo from your photobucket site really captures that better (although this image would improve from being cropped a little to get the frogs eyes out of the dead center).
This Iguana shot is good, but appears to be slightly oversaturated with color. It gives the photo "punch", but it doesn't look realistic.
Again you have cropped to put the subject (the eye) dead center in the frame. Try moving it to one of the "third" corners (search for the "rule of thirds" in google to learn about this compositional rule).
This uncropped iguana shot again appears to be oversaturated. Also this shot shows one of the dangers of shooting up into trees (a problem I often have with bird photos I shoot). The bright sky causes specular highlights which are distracting bright spots in the background of the image. They tend to take the observer's eye away from your subject. You could crop out some of them, but they are a constant problem when shooting up towards the sky.
This Tungara Frog shot is nice. I think the lighting is a bit harsh (too much dark background). You could lessen this by using some sort of diffuser on your flash to spread the light more evenly. For an on camera flash, you can make them out of any white plastic (like a milk jug) or even putting a loose tissue over the flash. Search online for homemade flash diffusers for some ideas.
Again, try to experiment with placing the subject (eye) off center a little.
One last comment about this photo is that Physalaemus are pretty terrestrial frogs, in my experience. Therefore placing it up on a leaf looks a bit odd (like photographing a toad in a tree).
With many of your shots, you don't have the eye as sharp as it could be. Whenever you photograph a critter, the eye is the key. If the eye is in focus, the shot looks good, if it isn't, it won't.
It looks like you could use a little unsharp masking of your photos. Nikon sharpens their jpgs a little more than Canon, Sony, or Pentax, but generally they need a little more sharpening during processing.
BTW - looking at your photobucket site, I can tell you saw some neat stuff and got some good photos.
Thanks for posting, and feel free to ignore any of my suggestions you don't like! 
Chris
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Chris Harrison
San Antonio, Texas
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