agreed with panamaxis and chris is me that pokemon are probably the main culprit, so this should be decided on a case-by-case basis (i.e. banning specific pokemon). my reasoning is a tad different though. basically I view any decision to ban specific moves as an unnecessary complication.
my basic premise is that a "competitive pokemon" consists of a species as well as legal moves and legal abilities. if you want to use a sleep move, then you have to use darkrai/smeargle/roserade; you can't just slap spore onto a sd garchomp and call it a day. thus, at its core, there is an inherent opportunity cost to using a sleep move -- it restricts the set of pokemon that you can use. spore and sleep powder are undoubtedly good moves, but due to this opportunity cost, their true value is dependent on the strength of the pokemon using the move. this means if a sleep move seems broken, it is probably because the pokemon using the move is overpowered, so our existing framework for pokemon bans handles it.
to put things in perspective, think about all the times in 4th gen that your opponent used an offensive team with a sleeper and you never got a chance to wake your pokemon up. or when you used smeargle, spored a restalk gyarados, and subsequently felt very silly. what about when you tried to wake up your pokemon, but instead let your opponent set up a dd/sd/sub? none of these imaginary situations depend on the mechanics change from 4th to 5th gen. in spite of that, none of us felt that sleep was broken in 4th gen... so maybe it is just specific 5th gen pokemon that are the issue.