1. becoming comfortable in the roles of the other classes helps you on your main as well
2. they don't get autistically caught up in their favorite playstyles and pushes and instead focus on the gaps/mistakes in your team's gameplay that are losing you rounds
3. discrepancy in the amount of time spent practicing
4. yes if you are friends
5. Do whatever you can stand to do long enough to practice, don't torture yourself
6. positioning and speed (to push/sack/fall back etc) - by main level you should be near fully comfortable with the raw mechanics of your class
7. I compare my dpm to damage taken, look at # of deaths, and look at the heal spread to make sure I'm spending the appropriate amount of time near the medic (class dependent)
8. keep in mind what the rest of your team is and isn't aware of, don't need to yap about a player in front of 4 of you but you DO need to call out unseen bomber, backcap attempt, damage that can be followed up on, etc
Bonus:
9. yes, in every element of life not just games
10. it's the one that I am the most comfortable/automatic on mechanics so I can focus on teamplay