OK, not all of the empty areas are accessible. In this example:
There is a large area towards the top left part of the map that is just green. Does it serve a purpose? What people are getting at here is that the map could be made smaller, enabling it to be more playable and enjoyable.
I don't understand what you are getting at here, but I can empathise with you on this one. I find it incredibly difficult to make a map that is not square. Practice makes perfect on this one I'm afraid. :(
This, again is mainly because of the un-needed blank space. Fill these up. Make them exciting. Use different types of grass too. (Incidentally, the fire red base has four to choose from.) :)
OK, I have circled some of them in the following screenshots:
Firstly,the main tile map error on the first map is the grass on top of the mountain. I don't really know what you have done, but you have not done it right. I believe there is a tile that has a mountainous edge on it. That is the tile you should be using instead of the dirt floor.
Actually, I quite like this map in a weird way. Although it is very simple, it is fairly easy on the eye. I have circled, however, the tree on the right. It just doesn't look right. This is by far the best map, just for the count. :)
The same problem exists on this map as the first. The hills are not using the correct tile base. There is also a error on the base of the lake, where the bridge does not link all the way onto the grass. This is quite difficult to do, and something that took me and Magneto20 quite a while to master. Perhaps look at how other people map these areas. Also, the stairs heading down, that I have circled do not look right to me. I don't know why. They just don't.
OK, the main bug-bare about this map is the roof's of the cave entrances. Although it might appear insignificant, there is a special tile which gives a nice rounded top to them, as opposed to the square, cut off look you have at the moment. Also, the mountain is very, very square. I have been told that the art of mapping is to give the illusion of diagonal tiles, while only having square, vertical ones. Try looking at how people like Master of Puppet's maps his mountains, or Deviation's indoor map a few posts up. Use these as inspiration, and practice makes perfect.
Yes. Tree shading is of the utmost importance. Although it is difficult (and I'm useless at this so I can't really help here I'm afraid) it just gives the map a little bit ore personality, and makes it look more professional.
Oh, and something else I have to point out. Flower placement. placing flowers in large blocks does not look good. In fact it looks terrible. Try placing them diagonally across from each other. I believe there was a tutorial on this by Sasquatchd00d, so it may be worth a look.
On the brighter side, the more you map, the better you will get. (Have a look at some of our original Septo Conquest maps if you don't believe me.) :)
Don't take this all to heart, all I am trying to do is help you improve your maps.
Good luck with the improvements. :)