Now I start by admitting the flies I tie are not works of art (and I greatly admire those that tie amazing patterns) they are functional and tied to catch fish.
So what do I look for in a good wet fly? A few things come to mind: size, colour and contrast, and movement. So when tying flies I choose materials that provide these fundamentals and are easy to use:
In terms of size I tend to use size 14 -18 hooks (I find smaller flies work well). Hook sizes frustrate the hell out of me, but that's another story! And I try to be sparing with the dressing often using just a few turns of thread and a hackle, as in spider patterns.
Colour and contract are important especially in clear water. I like to tie flies that are not too garish but have a small hot spot or a strand or fritz to make the fly more visible without being ostentatious!
Movement too is key is getting a fish interested and a nice soft hen hackle and sometimes a few tail fibres can achieve this. In deeper water where light diminishes and colours are of secondary importance movement becomes paramount.
The result is a simple pattern but one with plenty of interest to whet a fishes appetite.
So my advice is to keep wets simple but interesting.