In my opinion, the learning curve is as positive as cool for the player into a well designed game.
The " Hey, I can do this... and that also. Wow!. What if I do that.. yeah.. it works.. Damn, i do need to practice this cool thing more... Heheh.. And what if..?" factor is essentially part of the game's lifespan ( And mods, mods, mods.. the Celtic guy had a point there ).