I report rule violations and underperformers, nothing more, nothing less.
Riot says you can't do this. This means you can't report someone for "not playing defensive after a couple deaths", which is arguably a stupid idea anyway with the new comeback system awarding so much exp. You can't even report for not warding (which actually is pretty poor play, they're worth the money), playing bad just isn't a reportable offense.
To Riot: Copy my post, or just say the same thing and you will probably clear alot of confusion. I honestly believe that the majority of your players (and remember, the majority of players do NOT use the tribunal, so I'm not saying this gets punished) think that you can report for poor play.
Btw I know that Riot didn't count unskilled player for this mass ban but it still gives players the wrong idea which could lead to mistakes in reporting.