mine have been mentioned. proportions are paramount in importance, once you have a good sense of them, though, they become second nature. attention to detail includes managing your material as well as proportions and properly dubbed and tapered bodies.
I'll add a couple things that haven't been mentioned. I had a breakthrough when I realized that nothing involved in tying a fly involves any more effort than holding a pencil. take your time and take care in offering up yer material and making your tight wraps to achieve the result you require. it is not a race. finally, practice. every thing is difficult before it is easy and it only becomes easy with practice. which is prolly why I tie so many flies. I want to get good at it.
so what did I say? like six things when asked for two? then the first two I mentioned.
mgj