The biggest problem has got to be afk in champ ban / champ select, which assigns you a random. Not the "dc" blue / yellow caution sign. I mean just straight afk. Here is why i think it is the number 1 reason.
1. AFK @ champ ban = less bans, team gets upset, environment becomes toxic, someone queue dodges, OR no-one dodges and goes into the game with an already negative attitudes.
2. AFK @ champ ban = less bans, many "op" champs left unbanned, enemy team ends up with a champ your team feels is hard to beat, someone queue dodges or goes into game with negative attitude.
3. Bans champs but afks at champ select = random champ select, team gets upset because random champ doesn't fit team needs, someones gets upset and queue dodges or goes into games with negative attitude.
4. Bans champs but afks at champ select = random champ select, player is bad with the champ selected, feeds, or fails to contribute, causes negative attitude, increased reports in tribunal etc.
What i'm trying to point out is afk in the champ ban / select, leads to other issues that have been mentioned in this thread. If issues b, c, and d, can all be traced to issue a, then maybe we should start at the source, and work our way out. Instead of starting at the wide spot and working backwards.