For me it has to be very functional, and nice looking.
Water proof, not water resistant, but water proof enough that I can survive a decent deluge. Breathable enough to not feel clammy when it's just cold and windy out.
Long enough to keep my trousers dry, and so that I can stay dry when sitting. Trench coats, dusters, and ponchos are popular in wet conditions for a reason. You just need to be sure that it isn't so long that it becomes a tripping hazard, so probably to the knee but not much longer.
Thumb holes could be a nice subtle addition, but I'd prefer cutting the outside of the sleeve longer (possibly coming to a gentile point to look interesting) works just as well to keep the hands covered and is easier to put on and use with gloves. The longer flap also acts as drip edge to keep water from running into pockets.
I'd sacrifice a tailored fit to allow me the freedom to wear a sweater, blazer, or fleece underneath. If you want to carry a camera I expect you're going to have to make the same sacrifice, but perhaps a zippered front with a magnetic flap, that way you could partly unzip the coat and just pull the camera out when you want.
A hood that can roll up into the collar. I don't want the look of it most of the time, but it's very helpful when it rains.
Lapels that can be folded in and buttoned up to the neck when the weather turns fowl.
Pockets and more pockets. Not patch, but jetted with a flap, on the diagonal for hip pockets, and zippered pockets within the hip pockets so I can secure a wallet or keys and not have to worry about dropping something when I sit. I also like vertical zippered pockets on the chest, and with a flap or subtle sipper they don't have to be ugly. If the coat is lined, have the outside pockets go into the insulation so I can warm my hands.
Weight. Don't try to make it too light, your in the city not climbing a mountain. It needs some weight to drape properly.
On the style front Norwegian Rain has some lovely raincoats, but they aren't in the price range that you're talking about. http://www.norwegianrainshop.com/raincoat-single-breasted-classic.html