What Vilters said. Can't count the hours I spent manually deleting vertices from hair strands in Wings. Most often I could reduce polycount by up to 70% without any reduction in render quality whatsoever.

I'd like to see more realistic everyday hair instead of over the top fantasy stuff.

More period hair. Would love to have some "Belle Epoque" hair and some quality 80's big hair.

More useful morphs like "running" and "upside down" instead of a gazillion of ridiculous looking "styling" morphs.

Would pay double, nah, triple for some simple mid-parted long hair with a truly realistic upside-down morph.

In general, I'd like to see more realism. Transmapped hair can look very convincing if it's shaped correctly.

Most Poser hairs have good geometry, excellent textures and shaders, but really strange shapes.

 

BTW, it's quite easy to mirror a hair in a modeller. I think there are even python scripts out there to do that inside Poser.