You have two styles of fishing jackets to consider, the lightweight waterproof parka and the breathable wading jacket.
Lets look at the parka first. The better parka's are breathable, lightweight and roll up into a small bundle for easy storage. The main function is rain protection from showers etc but they are not a wading jacket.
These are good to throw on your bum bag when wading the flats.
Now the wading jackets or fly fishing wading jackets.
The wading jacket is a specialised jacket specifically designed for fly fishing/wading and should keep you dry when fly fishing in torential rain.
Firstly your wading jacket should be made of a quality breathable material similar to your waders. It also should have a waterproof zipper and a weather flap to keep you dry, neoprene cuffs, plenty of large pockets, fleece lined handwarmer pockets, and a full large size pocket at the back to store spare fly reels and your lunch.
Velcro cuff straps stops the water from running down your arm while casting. Be sure that the lining of your jacket is made from a non wicking mesh so it doesn't wick water while you are wading.
A large hood that is detachable. If you wear a broad brim hat you tend to not use the hood and then it fills with water, if it's detachable problem solved.
