Not banning certain equipment be it custom or official. Once you start the first example of a GM sponsored event that starts limitation on those things, everyone cites that as an example to push for the path that they want, mostly the GTB.
Item and/or skill limitations I'm fine with, but equipments/cards that one I'll say "no" right now.
(Noble triggered pvp tourney are exceptional of course... they get to call most of the shots afterall. If they want to ban certain equip, that's their choice.)