Sound is very different. If you don't use a dampener, a solid thump can suddenly sound very tinny.
For me, the thing that takes me most by surprise is the service toss. It's really quite strange to look up and not have a roof there. There are also little changes that shouldn't matter: where I play indoors the lines are very thick, but this usually isn't the case outside. Somehow this makes the court appear smaller, even though it's not.
The biggest adjustment I make to my game is the amount of spin, even in the slightest breeze. If you're with the wind, you need to use more topspin to keep it in. If you're against it, you'll find yourself swinging harder to combat it, and the extra bat speed often means putting more spin on the ball than necessary. Slice is very useful when you're playing into the wind.
Try to enjoy it. Tennis is supposed to be played outside.