It's really fun when you have a decent group playing together. My friends will often form a voice chat with each other and jump in, four or five of us together. Then it's awesome. You can communicate efficiently, plan things out before hand... it's a major help.
The griefers are the ones who ruin it. And, I mean, you have griefers in every online game, but LoL, being so team-based, gets it particularly heavily. If one person on your team is being an idiot it is entirely possible they will cost you the game. That I can understand is frustrating, but, especially on low levels where the newbies come in, you can't go in expecting everyone to play well.
It isn't fair though that the LoL "fans" expect everyone to have the metagame memorized down to the tiniest detail (my friends like to troll these players by deliberately picking odd but effective builds, watch them complain about "that is not how to you play that champion", and then do well in the game regardless). And even then, some people will give what they think is an "obvious" signal and then get angry when another player doesn't recognize it. I get this sometimes even with my group of friends, as they are all far more experienced than I am.
Funny thing about always playing with a team who is better than me means my skill level is in a strange place. Anytime I play with them, I'm not at their level. But if I try playing at my own level, in just a random game, I'm usually slightly better than the rest of them. Getting beaten all the time by superior players has imparted some extra experience to me, it seems.
tl;dr: FireSpawn is correct: it's most fun when players don't take winning and losing so seriously.