-Moody raises the raised stat by two and drops the dropped stat by one. That means that you net a +1 to stats every turn (which is good in the same way speed boost is good, except less predictable). At least in theory, that means that a physically attacking Bibarel at +6 attack, +6 evasion, -6special attack after just six turns.
-Sleep isn't all that gamebreaking, and that's why they let you do it. But if your whole team is asleep, you literally can't play the game, and what's the point then?
-I believe the Swagger clause was created to avoid buffing an ally with a persim berry - thus doubling their attack even though they don't get swords dance, allowing them to attack on the same turn as they buffed their attack (reminds me of Mega Kanga), and activating unburden if they happen to be graced with it.
-SmashPass is a legitimate strategy, as is Speed Boost passing. Part of the reason boosting teams have any risk is that if you're forced out, that's it, your stats are gone. That means that the momentum can be broken. However, if all of the members can pass to each other, it removes that balancing factor that makes stat boosters bearable. If everyone can pass to each other, it is very overcentralizing.
-Mega Lucario. I wasn't active in the community when it got banned, but I think it's for high attack with adaptability making it able to dish out high damage even on things that resist it. I personally don't really feel it was gamebreaking (too frail), but it got an overwhelming majority vote to be banned, so I think I'm missing something here.
-Mega Mawille. I agree here.
-Mega Mence. A total monster that absolutely needed to be banned. Seriously. Aerilate is a stupendous ability, which allows it to fire off double boosted (x1.3 from aerilate and x1.5 from STAB, resulting in a net 1.95x power from normal moves). That means it will absolutely annihilate using a return, since it's over a hundred base power with no drawback. Additionally, its mediocre speed was kicked up to a level that makes it outspeed most anything, especially after a dragon dance. Its defense got a huge boost, and its attack also got buffed, not that it needed it, what with it already being powerful and having aerilate. Ludicrously overpowered, definitely a good ban.
-[Mega] Blaziken. This one's interesting in that the mega isn't really very different. However, with speed boost as an abillity, after a swords dance, it is a ludicrous threat. As long as you force a switch when you send it in, you have the power to just instantly sweep. Since the mega isn't strictly necessary, you can still have a mega on your team that isn't blaziken
-Aegislash deserved the ban. It has 150/150 defenses and 150/150 offenses if you use it well. Its low speed works in its favor, since it can take a hit with its super defense, then switch to offense and hit with its super offense, all the while crippling physical attackers with king's shield (a priority switch back to defensive).
-Greninja's protean meant that not only did it get coverage, but it had mind games as well. It has the moveset to hit everything really hard, as well as the speed and attack stats to back it up. Additionally, it could switch its type, so it could switch to a resistant type for any incoming attacks. Remember, Greninja wasn't uber until ORAS came out, because it got the few moves it lacked to hit everything ever.
Overall, Smogon makes the right decisions regarding these things, and every decision they make is for a sensible reason.