I think of it like this.

When i raced cars, i looked 15-20 feet in front of the car, and reacted to something as it happened.

In planes, you have to look 100feet or more ahead (or at least think it) and react to things that might happen before they happen.

Also, in planes you find that you use all of your senses more.

Obvious sight...sound constantly (listening to the motor, hearing it try to lean out before a deadstick, also listening to your surroundings for other people)

Smell (when your working on the motor after a run, the smell can tell you alot about how it ran)

Constantly "feeling" the wind and thinking about it

And of course..."taste" sweet success on every perfect flight!

The major difference, as some have said...but not quite, is the competition.

With cars, its hard to have fun outside of a competition. So everyone is pretty much always competitive, not a bad thing neccessarily but it does tend to interfere with the relationships/freindships that could be formed. At the flying feild, competitions are just "extras" and stricktly for bragging rights (might be a prize involved at funflys and stuff...but its still really about the bragging!)

Cars seems like so sweet to me (carpet racing for a LONG time) untill i flew...the dat after my first flight i sold all of my cars except my old RC-10T2 that i keep around for the occasional bashing (and something to play with with the plane batts are charging)