I might as well have your post here and just expand on it because I was going to say exactly that.
Things I want to expand on:
Kings Shield: Perhaps the most frustrating move, up there with scald and swagger(maybe not swagger lol), this move lets aegislash completely stop and destroy pokes that would have otherwise been solid checks, and turns the metagame into a glorified game of flipping coins. Pokemon like choice band talonflame, just might as well switch out because a -2 flare blitz only does about 60%, and then it gets killed, which isnt a good trade. This leaves aegislash vulnerable to only three types of attacks, indirect physical moves(sacred fire being the only notable one), ground moves which get blocked by air balloon, and special moves. So pretty much in order to kill aegislash we need to pop its balloon(which is usally a suicide mission, or a nice prediction), or nail him with special attacks most of which are dont hit him super effectively, and he can sponge them up with his 150 SPDEF STAT.
Variety in Sets: As if King's Shield didnt force enough 50/50s, with all of aegislash's sets even the most reliable counters can be reduced to rubble or praying that they win a 50/50. Ill tackle the most common counters and then post calcs about how different aegislash sets can handle them.
Mandibuzz: This was the number 1 aegislash counter for a good portion of the metagame, but then sub toxic aegislash became popular and all of a sudden mandibuzz is irrelevant.
4 Atk Mandibuzz Foul Play vs. 252 HP / 0 Def Aegislash-Shield: 66-78 (20.3 - 24%) -- possible 6HKO after Leftovers recovery
sub toxic aegi is free to sub and toxic away until mandibuzz is dead, and aegi comes out of the exchange behind a sub.
Bisharp: While Bisharp was always considered a shaky counter, because switching into a sacred sword was so terrifying, no one could argue that bisharp was an excellent aegislash check, but now there is a common set that runs max speed and is capable of outspeeding and OHKOing bisharp. So now forget about pursuit trapping bisharp has to decide if he wants to sucker punch or knock off and if he chooses wrong that could be the game.
Heatran: Remains perhaps the most common aegislash "counter" because he can sponge up shadow balls with relative ease, but the same set that scares bisharp also is able to outspeed and 2hko heatran, which means even heatran has to know what set aegislash is before he switches in.
Let me point out that these^^^ are aegislashes best switchins, period. (barring meloetta and braviary, but those are gimmicky imo) The reason we banned lucario and now deo-s, is because you had no idea what set it was running and that poke had the ability to run a set that would destroy a poke that countered the common set. This combination is too strong and that is why we ban pokes that possess this quality.
I have more to say but I have to go so this will do for now, Ill just start replying to counter-ban arguments when I get back on. I hope my clear position on wanting to ban aegislash doesn't discourage people to question any of my points, I am open to all viewpoints and reserve the right to change my opinion if there is an argument I find convincing. I look forward to seeing a healthy debate about this pokemon because I feel it has the best chance of staying opposed to the previous suspects.