- Is Sleep broken/too powerful or simply another viable aspect of Pokemon? Is it overcentralizing?
Short Answer? No.
Long Answer? Sleep is just another way to hinder your opponent, just like Burning physical attackers or Paralyzing fast mons, or even luck based strategies like ParaSwag. Overall, they all do the same thing in that they mess with the opponents and create free turns. And where Sleep is a little more powerful overall since it completely shuts down a mon, it's not permanent like Burn/Paralyze and doesn't have a set amount of turns, making it less reliable overall as a strategy.
- Does it restrain teambuilding too much? How much preparation does a team require to combat sleep?
A lack of sleep clause punishes badly built teams, as said famously by Mizuhime many times in IRC. Sleep is blocked by the same moves that block any other status. Taunt, Safeguard, Fake Out, etc blocks or otherwise prevents status or their respective abusers. If you prepare yourself for status in general, you will be fine against sleep. If you're really paranoid, you can add some Grass types to your team, add a Lum Berry somewhere, add Sleep Talk to your Guts Conkeldurr. There's plenty of ways to deal with sleep that are already built into the Metagame.
- Does Sleep limit or increase diversity of the metagame?
If anything, it increases it. It makes mons like Jumpluff viable, and brings up Darkrai and Amoongus, as well as giving us reason to use Breloom again. Overall I see it positively affecting us.
- Does Sleep have an overall positive/neutral/negative impact to the Doubles metagame? How does Sleep affect the metagame?
Neutral. It just adds another later of strategy to an already very strategic Metagame. While someone brand new to the Metagame might find this a bit daunting or even intimidating, I see it bringing our skill level up overall, as it makes is think more about our teambuilding, as well as making us think a bit more while playing. So while it may make it a bit tougher to get into at first, overall it raises our skill level and makes us better players in the long run, and that's the goal of this whole thing right?
- Does the opportunity cost of using sleep balances out the rewards from sleep spam?
I don't think Sleep Spam is a super rewarding strategy overall, as it's super risky due to the fact that sleep turns are random and never a guaranteed amount. It's also handily shut down by any good team that carries Fake Out, Taunt, or Safeguard. So overall, yes; I definitely feel like the cost of using such a team is balanced by its inherent risk.
- What are the different checks or counter strategies to combat sleep?
I think I've listed all of them already, but Disruption (Fake Out, Taunt, Rage Powder/Follow Me into a Lum Berry, etc), Safeguard, etc all trump Sleep. Focusing the Sleeper is also known to work. Basically, anything you'd use to shut down any other annoying mon can be used to break Sleep abuse.
- Are there any suspects independent of Sleep that you want to raise attention to the council?
Freeze Clause pls
On a serious note, I still feel Mega Kangaskahn needs a serious looking at. It's so easy to throw it on onto a team and it does so much damage. It has a limited number of set options but it has enough variation to keep an opponent guessing until the set is revealed. Personally I find running Crunch over Sucker Punch to be just enough of a surprise to kill the opponent's (usually just a single) Ghost type which opens up the whole team to Return/Drain Punch (PuP).
The other thing I've noticed is that almost every single competitive team has Heatran on it. That's not to say it's too powerful, but it's something to take notice of.