In reply to madyarra:
I think one of the biggest factors is the ability to read a problem - not having brightly coloured hand holds, or obvious foot holds can make a massive difference when you first go outside.
Also, you find that outdoor problems tend to be more difficult to "Unlock" simply because the technique needed (drop knee etc) is not obvious.
On top of that, there is the mental side where rock doesn't seem to have the level of friction you are used to, and the mats seem further away and less confidence inspiring.
Keep at it though, and you'll find it much more rewarding