Okay I doubt anyone here actually cares for a solid argument, but I'm a badgeholder on Smogon and I think I can properly explain what is the problem with Aegislash.
Just look at a list of good OU Pokémon. Clefable (offensive), Pinsir, Scizor,Tyranitar, Venusaur (offensive), Dragonite, Latias, Latios, Terrakion, Scolipede, Gardevoir, Medicham, etc.
What's with the list of these Pokémon? They all have to dedicate one move on their set to take care of Aegislash, which means they only have 3 moveslots left. Take Terrakion for example, last gen it had a very solid Swords Dance / Rock Polish / Stone Edge / Earthquake set, it can SD vs stall and RP vs offense, and threaten the opponent with its dual STAB. This set sucks this generation, Terrakion must carry Earthquake because it's not able to hit Aegislash otherwise.
Same for all other Pokémon on that list (and more, some Pokémon even got unviable because of this). Medicham is forced to carry Fire Punch, which due to King's Shield incredibly broken Attack-dropping side effect might even not be enough.
Talking about King's Shield, a lot of you are saying it is 100% predictable. It is not. Talking about high-level play (which bans and suspect tests are based of), it is just a bunch of 50/50s. A good player doesn't just use King's Shield every other turn, it makes it somewhat unpredictable.
Oh and another thing, Aegislash is not only amazing defensively, it is also incredibly hard to switch in on. There is pretty much no Pokémon that 100% counters every single Aegislash set. Specially Defensive Hippowdon or Mandibuzz? Loses to SubToxic (or SD Head Smash for the latter) and are unviable on offensive teams. Ok, how about some offensive checks then. Landorus? It can't switch in because Shadow Ball + Shadow Sneak has a decent chance of KOing after Stealth Rock. Bisharp? Can't switch in either because Sacred Swords OHKOes.
I could go on, but I think you get my point. Feel free to ask questions and I'll reply though.
