Ask HN: Terms when joining a company as co-founder

I've been involved with building several successful products for a company that now wants to break those out into a new company. They've asked me to join as a co-founder. The mother-corp will make a substantial investment and will hold the majority of the shares -- the rest will be split over the co-founders (which includes me).

My fears are: - to be treated as an employee (not able to browse the bookkeeping, or not in the loop for important decisions) - to have shares that I cannot 'use' (heavily diluted, or unable to sell) - that there is some obvious mistake I'm making, only to find out several years down the road.

Advice is greatly appreciated.