Skip right to the giant letters and bold words if you don't like to read lots. 
First off, I have absolutely nothing against 'noobs' (or to put it more accurately, simply 'inexperienced players'). I was a noob. You were a noob, we all sucked at one point and fed enemy teams wonderful items and upgrades. I don't mind a new player (one with very few total games under his/her belt) coming into a game of mine. I like to teach and coach and give friendly advice in-game, even to opponents.
Secondly, I do not consider myself a 'pro' or some other label that indicates I'm proud and think I can pwn you. But sometimes I'm in a competitive mood and want an even, challenging game. Thus I set out to host one. I usually make game titles like "US 3v3 *Pro's only*" or "US 3v3 NO NOOBS" just to try and make my intended point clear and simple. Then I sit and wait in the game lobby to see who joins. And usually, unless I'm sick and tired of spending time doing it, and decide to leave the teams' balance to fate (always a bad idea), I alt-tab to check people's stats, to make sure that (whatever their win percentage) they at least have 50 or so games played. I want a fun, interesting game, not a slaughter for one team (even if I'm on it; that's no fun either). Then I observe things like this happen:
A. A player joins who has played 10 games or less.
B. A player joins with around 20 games played total, 17 of which were losses, and claims to be a 'Pro.'
C. A player like either A or B joins, I tell them why this isn't the game for them, they say nothing and don't leave, or argue, and I am forced to boot them.
D. A player like C joins, I tell them why this isn't the game for them, they say nothing and don't leave, or argue, I therefore boot them, and then they come back yelling at me for what a jerk I am.
So I ask my fellow players and hosts this: WTF?
I've talked to fellow waiting players in-lobby, and below are some things I've heard as explanations. Let's discuss whether these are true. Er, you can discount the third one, I'm pretty sure that's not generally true. ^^
- People don't read game titles, only keywords (like 'EU' and '3v3'). Is this true? Please tell me this isn't true. I've heard this A LOT.
- There aren't enough games to choose from, so people join a 'pro' game anyway, because they don't care about a challenge or losing, they just want to play.
- People are bitches. This one is more a joke between other waiting players than a serious explanation, so someone please give me a better theory as to why people rejoin my game and start bitching at me for kicking them when I first explained to them why I and the other players in the lobby would rather they not play here.
What gives? Let's share thoughts and maybe we can all understand our fellow players a little better and be better and more efficient game hosts.