Hi there...i only play elves so i cant really say anything about other races...
1. When there is rain, there is no way that forest can burn and fire can spread
2. Autocast on units that can heal or cast downpour (on fire)
3. Elven wells for regeneration really look bad, (water streaming from a tree???)
4. Move and attack should be default option
5. Improve dragon animation while flying, units look awesome when moving and attacking but dragon is meh
6. Make heroes better visible in combat, i cant tell the difference between him or some other unit when fighting starts.
7. Selected unit circle (lower left part of the screen) is too big and information displayed is confusing (not arranged good) takes a while to find what you are looking for.
8. Position of chat window should be in lower left corner (where unit selection is now). That has become a standard in mmo games. This way it always gets in a way and at least in my case it is always closed.
9. Icons for building menu and unit icons (abilities like heal, change stance) aren't good for the elves. The contrast is wrong. I know thats not a priority but im a sucker for shiny things. Considering how good the elven tree buildings are made the icons dont do them justice.
10. Option to stop producing selected resuource. I have full wood, full food, but cant get stone, cause total resources are capped. I would like to stop producing food when it reaches 2000, and wood when it reaches 2000.
11. Different skins for different unit levels, like adding gold streak when it reaches level 5, weapon shine when it reaces level 10... (again shinies).
12. Elven forest is kinda empty, seems like nothing lives there, no birds, no animals, no ancient ruins
13. Decorations for cities...statues, fountains, market, city feels like military camp not a city.
14. Treants are ugly they look like tree stumps (i know i have been repeating this since i joined the forums, and it isnt easy to change a unit skin, but it has been bugging me like hell :-)
15. When in home city i would like that my unit regeneration is increased or at least set to 1hp/min regen, i have a wounded treant that cant reach a regeneration fountain cause he is to big and my enchanters cant heal him cause they heal is 80 hp per click. I would need to spend 2 hours just clicking heal.
And i liked the idea from Aeil about range indicator, that would be awesome.
Here are some links for icons that you can take for inspiration or maybe use. (if it is not allowed to post external links please remove them)