(Time to do a little catch up...)
Day 18: What innovation could RPG groups benefit most from?
I'm not sure how to answer this one, so I'm going to have to pass.
Day 19: Best way to learn a new game?
Find a GM who knows the rules, can explain them clearly before and during play, and who can present an interesting adventure regardless of what the system is.
If you're going to be playing for more than just a few sessions, then you really should acquire your own copy of the rulebook and read it. Your GM, no matter how good they are, can't always cover everything you'll want or need to know, so being able to look up rules yourself will help you master the game faster. This will also free up some of the GM's time and energy for other parts of the game experience besides teaching.
Day 20: Most challenging but rewarding system you have learned?
I've been gaming for long enough, and have good enough reading skills, that I rarely have to struggle to learn a RPG. Those few times that I have, it's hurt my enjoyment of the game. The games with the longest learning curves that I've enjoyed enough to play long term were probably D&D Third Edition and Earthdawn.
(Days 21-22 coming soon!)