Advice for a new photographer to increase his skills

1) If you use the 70-300 @ 300 most of the time, then perhaps you need a 200 - 400 lens or ..... get closer. Your tools should be utilized for what they were meant for. In other words, you might prefer a fixed 300mm lens which would be an improvement over using a 70-300 @ 300 and not utilizing the rest of the range. There are always compromises, so what you are giving up to have a zoom might give you more to have a fixed.
2) If you are adjusting the overall brightness then you are not using the advantages of having digital which means that adjusting overall is fine as long as adjusting a section isn't needed. If you are not adjusting a section and ONLY adjusting overall, then you are staying in amateur mode and won't be able to progress. Photoshop is intense and takes years to learn. Start with Photoshop Elements for under $100 and start learning it. VERY necessary. Otherwise, go to film and learn about neutral density filters and developing in your own darkroom. One way or another, in order to progress beyond amateur means ....... progressing and doing more then amateur point + shoot.
3) Metering and focal point is beyond critical, it's necessary. Central spot is correct, but move that spot to where you want the exposure AND the focus and then hold that exposure or focal point.
4) You MUST be on Manual except for basic point + shoot pictures. Using any automatic mode does NOT give you the advantage that you as a photographer must demand.
5) I DID know what you were going for with the lizard (but missed and am still missing the political implications but it is imperative that we NOT discuss ANYTHING political! PLEASE!) but it needs not to be so subtle. Subtle is fine for certain pictures, but not this one. Fill the frame with your picture.
6) Depth of field needs to be more in your control. If you need the shallowest DOF to make the best picture and you need 5.6 or 3.5 then THAT is what you need. If you want the shallowest DOF then YOU must create the shallowest DOF and not rely on the camera's automatic features.