Some of the below is subjective and may not be true but just my experience of Singlespeeding over the last 18 months.
You will learn to blow out of your arse,
You will ache in some funny places,
Your legs will look like a blur on occasions,
Gurning and swearing on climbs is normal,
Your upper body and legs will develop new bumps in them called muscles,
Girls could become pregnant when you ride past them,
Growing a beard will feel natural and be a good idea despite what the missus thinks,
You will be quicker than your friends with gears,
Real Ale will taste better after singlespeeding,
You will get fitter!
You won’t want to ride much else.