This is a strange coincidence. I downloaded and started playing League of Legends yesterday, literally. Well, to be honest I have played it once before and I have played a couple matches of other MOBA games. But for all intents and purposes: I am a clueless noob. (And for reference, I have picked Ryze as the champion I'm focusing on right now).
Things I have a major problems with:
Items - OH GOD THE ITEMS! Putting in a bazillion items with their individual stats and expecting the players to do the research outside the game on what they should buy is just stupid freaking design.
Jungling - I have a hard time grasping the mobs in the jungles between the lanes. I sort of understand the basic concept but some further clarification for a beginner would be nice.
Adjusting tactics - What I mean by this is that you're (apparently) supposed to adjust your tactics (in terms of gameplay and itemization) according to the situation on the field and the heroes you're teamed up with and against. What makes this problematic for a beginner is the incredible number of heroes.
The community - Yeah, I know there's not much to teach. Just trying to be calm and collected and not baiting when people insult you is the way to go. I'm pretty zen when it comes to this fortunately. But I must say, MOBA communities are the cesspool of the internet. ...well, after 4chan that is.