Snowy Egret in Mallorca by MikeRC

Technically speaking I can't see anything wrong with this photo at all, at this scale on the web. You've even used the -2/3rds exposure compensation to help counter the brighter parts of the white bird from overexposing (the whites along the back look strong white, but I don't think they've overexposed - histogram on your computer should be able to give you a more definitive answer, but they don't stand out to me at all).
ISO is where I'd start for wildlife - 200 is a good base to work from; though on dimmer days 400 is normally very usable on pretty much all cameras without worries. Shutter speed is certainly fast enough to freeze the motion (and has done wonderfully) and is also more than fast enough that any handshake should be of no concern (at 1/1600 at 300mm VR, strictly speaking, won't be having any effect on the hand motion blur in the shot).
Aperture - well you've been reading and chatting in the forums about the aperture and sharpness, esp with regard to the 28-300mm so I'll take it as given that you already know to try shifting to a slightly smaller aperture to get a little more sharpness out of the setup.

Having done no editing work at all its a good result - myself I'd say sharpening (esp after resizing) would be needed on the photo, but other than that no areas appear in need of work. Some burning around the whites might be in order if they are just blown - though if you shoot RAW I'd be tempted to process the RAW twice - once normally and once for the highlights (same settings as for normal, but then slide the exposure slider until the whites are more controlled; but not too far that they look "dark") and then blend the two images into one with layermasks in editing.

However I get the feeling that the result you've gotten has left you feeling like something might be wrong, that something is lacking that you'd prefer different. If you have some examples taken by others that you'd like to emulate if you could link to them it might help to understand where you want to head - what you'd like to be able to create with the camera.

Funghi by VonQ

I really like the idea of this shot - good lighting coming through the mushroom and getting a good low angle to show that aspect in the shot! The bright and colourful background also works well to offset against the more drab colourings of the main subject.

Lighting on the background areas though seems a bit harsh and I guess that is a result of slight overexposure in those areas so that you could expose under the darker mushroom better.

Its something that in the field I think would only be solved with 2 exposures and a tripod to steady the camera so that you could then blend the two together (one for the mushroom and the other for the background) into a single composit shot - though its not something easily taken in the field if your working without a tripod.

As it stands you went for the mushroom to be exposed correctly and in this case I feel that it was the right choice to make - well done!

Light on a Lady by Cormy

I've been looking at this macro shot for some time, even looking at the large version, but something seems off to my eye. It might just be that this butterfly has vastly different colourings to the ones I am used to seeing, but if you look at the wing closest to the camera around the shoulder, lower bpdy and in some areas back along the length of the wing there is a really odd colouring - the like I normaly see when I pull the levels or curves lines a little too much on one direction.

However loooking at hte top of the wings they appear to be blurred in motion so it might be that this along with an unusual colouring is making me see things that arn't there - this is also a little apparent as the markings don't appear anywhere else on the photo.

Its an interesting puzzle to me in a good macro shot

The Lizard by Andrew_Hurley

I do like the colouring in this shot - have a very real feeling to it which we don't often see much in digital shooting as was seen in the film era

However it seems that your focus missed his head and hid the middle of his back, if you can in such shots you really want the eye of the animal (which is looking right at you in this shot) to be in focus, even if nothing else is. I would have either zoomed/stepped back to get the AF on his head (using a single centre AF point only) or used manual focusing to correct it.

Also the lighting has been tricky for you and very harsh. I have done an edit where I used the highlights and shadows tool in photoshop elements; I added light to the shadows to bring out his head more, whilst also adding a lot of shadow to the highlights to lower the glare in the shot to a more acceptable level. I also used a layermask on this (though you can use a selection wizard instead) so that it only affected the insect and the rock he was on and not the background, which does not need the added lighting.

bird in flight by claytonbell

I can't tell what settings you were using, but I suspect a wide open aperture combined with a shutter speed that was as fast as you could go - with a little underexposing as well for more speed. It looks like your focus is good and that you captured the moment, but even with underexposing for a faster speed (assuming this) the shutter has just not been quick enough.
If in this case again I would say flash or a very high ISO are your only real options and of the two I would reach for the flash myself - rather than high noise from a higher ISO.

Its a tricky thing but birds in flight need a lot of speed - its no trick that many bird in flight shots are taken in very bright weather - its because dimmer weather just does not give the light needed for those faster speeds.