New Champions in Ranked Play
So, as everyone knows, new champions are pretty buggy. When Azir came out, he had so many bugs that he was almost unplayable. Vel'Koz had some issues after his initial release. These bugs can be a very big problem, especially in ranked play. Of course, new champions have to be tested and played to find potential bugs, even after the PBE release.
Me personally, I hate to see new champions (especially buggy ones) picked when I start a ranked game for multiple reasons. For one, most people don't really know how to play new champions. That results (A lot of the time) in feeders and flamers thinking that because they're playing the newest champion, they're better than everyone else. Secondly, new champions can be extremely buggy, enough so that it changes the outcome of the game.
In my opinion, new champions should be disabled strictly in ranked play for at least a few weeks after their release, to give Riot the chance to straighten out any problems that champion might have, as well as give the people who want to play that champion in ranked time to actually learn the champ.