Some random questions about LoL ( the game and the community) from new LoLer

[Sallomy]

New LoLer here, just got to lvl 8. Just have a bunch of random questions about LoL I am curious about. If you have time, feel free to leave your answers

• Does LoL have the largest Dota community? What about compared to the original Dota? With blizzard releasing their Dota, do you see people move away from LoL to join the battle.net ladder?
• I always heard there are lots of BMs with the original Dota. In my (very) limited experience, ppl on LoL tends to be nice -- maybe because i am at such low level. But in general, is that one of the attractions of LoL versus others?
• When you play a random match with others, do you generally pick the character based on the team's needs or just one you like to play.
• About Katarina -- read somewhere that she's bad at high level cause her ult. is easily canceled out by cc. Your thoughts? New players like me like to play her cause the damage she outputs. So in your opinion, at what level i may have to give her up (that is, she's so ineffective in a team match, that most ppl know how to counter her)