i'm with Caorach, but sometimes a waterproof outer is useful/needed - i'm thinking primarily of the times you're out in the open in foul weather and not doing much for hours on end...
fixed, grown-in hood, it needs to have a really strong wire in the peak so the hood keeps its shape in wind/rain, and it needs to be adjustable around the face and at the crown.
handwarmer pockets, and as long a front zip as you can manage while also having a decent front pocket. it needs to cover my bum even when i'm sat down - it also needs to be long, if perhaps not quite as long, at the front - you could have 6 inches of poppered/zipped side opening to make it easier to sit down while not having to move the whole thing up your body.
pit-zips.
no neoprene at all! neoprene is for wetsuits, its vile stuff to wear in any other context.
if you can't produce a smock with a really, really breathable membrane (high-end Gore-tex etc..) for a particular price point, please don't try. far better to use something like waxed cotton or ventile - the worst, worst clothing disaters are not getting wet from the rain, they are sweating like a Secret Service Agent at Donald Trumps inugeration...