If the weather is at all questionable, an absolute essential is a plastic trash bag. The two biggest threats to keeping warm are water and wind. A trash bag can impede both, if loudly, by simply putting it over your head and poking holes for your head and arms. If you're a good samaritan, pack two. Resist the temptation to skimp on clothes.
Some riders compromise by bringing an old sweatshirt and tossing it midmorning.
Don't try anything for the first time. The new shorts may chafe or bind. The new energy drink may make you sick. The new sunscreen may taste bad once your sweat makes it drip. The new saddle may feel unbearable after 150 miles.