Constructive Criticism, Especially at Lower Levels
Be an opener of doors for such as come after thee.' -Ralph Waldo Emerson
We all started somewhere, and if we're going to do justice to the people who helped us move up the ladder, we have to start by paying homage to our roots. If you see a player having a bad game, or who clearly doesn't grasp the fundamentals of the genre, try offering some constructive advice. If you do so in a civil and friendly manner, it's likely that they will be receptive. Oftentimes they'll be downright grateful that somebody took the time to let them know how to improve instead of yelling at them.
Never get frustrated by an inexperienced player's performance. At some point, you were just as green as they were, even if it was the day that you downloaded the League of Legends client. Have a little patience, and try and help the player step up to a level where both of you can enjoy the game. At the same time, don't be discouraged if they aren't receptive. Some small percentage of players will get hung up on the notion that they don't need anybody's help, and, no matter how politely you try to lend a hand, they won't want to hear it. That's no reason to give up on the rest of them!
I was reading the summoners agreement, and found this little gem. As a person who has smurfed for the heck of it (I'm not good enough to do it legitly, just wanted to not drag 30's with my newbie buddies) I've seen these poor starting players get yelled at, demolished, made fun of, and demoralized. I understand a lot of us get frustrated at that one person who can't cs (especially under turret) or the other one that wards, but doesn't have the map awareness to see a gank coming. Or builds that ridiculous item that makes no sense (adc Anivia anyone?).
Rather than YELL at the like "WTF ARE YOU DOING THAT'S SO WRONG YADA YADA" (Especially you smurfs out there!) try something like, telling them how to cs under turret. Maybe warding further down the river so they see things sooner. Save this spell so you can trade better. Build this in that situation. Add a smiley to the end of your statement, so they know you're there to help. If you have time to type, explain why something is better. Nurture these players as they are the ones that help keep this game alive. Without a new influx of players, this game will eventually die.
If they don't listen? Mute em. They don't want to learn; while you can lead a mule to water, you can't make it drink. However, don't argue why they're wrong. One of these days, they'll realize that they're not as good as they think they are.
If you have trouble communicating things to players in a positive way, let me know and I can give you examples of what is or isn't acceptable. Basically, your statement should be free of hostility and negativity, and filled with encouragement and constructive feedback. If you can't pull this off without sounding like an (forgive my french) @$$, then criticism is not your thing and I would suggest refraining from it unless you're with friends that can handle it.