Prelude this with a request that everyone who chooses to participate, please keep it civil. Attack the points, not the person.
The main reference point I may use will be the recent banning of a player who went Rengar top with cleanse/ghost. This post isn't necessarily about that specific case, but it is recent and easy to use as an example.
Bans, hovers, and picks, oh my:
1. Bans come before picks. Therefore you can ban whatever. Yes, it might be a dick move; but it isn't technically against the rules. It could count against you in a manual review though.
2. Pick Order still is a thing. If top wants Sion (and has been hovering it since beginning), but I want Sion as support (not hovered at all); I can take him as support if my pick priority is before top. Again, a dick move, but not against the rules. It could still count against in a manual review though.
Abra Kadabra Alakazam! (Summoner Spells):
As a player, you can theoritically take any combination of summoner spells and potentially find success. There have been certain ones that have become a staple in being norm though. Typically you see every champ rock Flash. However, there have been and are still cases when this is true. Shaco as an example may take smite and ignite when jg. No combination is strictly forbidden.
Doesn't mean some combinations are free from infamy though. Cleanse with Ghost is nicknamed (I believe) Disco when paired together. Does that mean every circumstance with this combination means troll? Not at all. If you have never used that combination before for a certain champion, it might look suspicious though. Swapping immediately to an unusually combination without knowing enemy team comp also may raise a flag.
Lack of Communication:
Should you be forced to communicate with your team? Yes and no. If you are going to do something unusual, it is important that your team understands what it is and why you are doing it. This actually can be talen back to the Smite Support Singed case. It wasn't so much for the strategy that he got punished, but more for not communicating what was going on. There is a difference in talking with people and talking at people.
Giving up with mindless pushing:
This is probably the main in game one that people debate about a fair bit. If someone has given up and just begins to afk farm/push. Not warding or caring if they get caught out. They are still playing the game, and pushing might get the win. This also can come up in open situations. If your team opens mid and you simply go to a lane to push to avoid afk, is one trolling?
These are mainly my thoughts and opinions on the matter. Care to share yours?