There are no difficult skills that are unique to AWPing.
You need to have a fast reaction time, good decision making, gamesense, etc. but this is true of all weapons.
Mechanically speaking, the AWP is objectively easy because you only have to hit a large zoomed in torso instead of a tiny head in the distance. Essentially, you are paying extra for easier aim duels.
If you want proof of how easy the AWP is, just go on a casual server and count the number of AWPers. It's the most popular casual weapon for a reason.
That said, is the AWP imbalanced? Not really. The price tag keeps it from being abused too much.