When learning the system, it's only a 2 or maybe 3/10, provided you only want to know enough to be able to play the game. If you want to take the time to figure out all the nuances of the game then it's closer to 4 or 5/10.
The challenge posed by combat is entirely dependent on your DM. In the game I'm currently playing, the DM plays the monsters to the best of their abilities and doesn't fudge rolls, either to help us out or make things harder on us. It makes for an interesting game, and the challenge is about 9/10.