While any pokemon with just Aegislash's stats or its ability probably wouldn't be as potent, these two combined with the typing makes Aegislash insanely powerful without much risk, and is one of the times where having lower speed is better:

Let's start out with Aegislash being sent out, which is always in Defensive Stance. This means gigantic defensive and specially defensive stats, along with only 2 weakness. Couple this with his speed means that just about every threat he could have outspeeds him, and since he only changes stance when his move executes, he has sky high defenses until he attacks, and then he has sky-high attack stats. Very few pokemon actually go slower than Aegislash and can actually do anything to him, the only one coming to mind being Escavalier, and even then it's not very good anyways, especially with the constant threat of King's Shield. If the Aegislash is running Weakness Policy, even its counters can't do anything to it, as it can easily just King's Shield, Swords Dance as it gets a free sharp boost to both attack stats anyways, and Shadow Sneak its way to victory. Or, alternatively, it can just use one of its STABs to make a dent in most pokemon, King's Shield, and repeat.



Really, the biggest problem is how King's Shield has Priority like Protect. This means that Aegislash can easily check just about anything, and even predicting the King's Shield and burning it is a crapshoot, as it could just as easily be running either a mixed or Special build, not to mention that thanks to its typing, Aegislash is immune to every form of Priority attack bar Aqua Jet and Ice Shard, both relatively uncommon and barely doing any damage to it anyways. Really, Aegislash is just too versatile, too potent, and too good. The only trues weaknesses I can think of for it is Spikes, Sucker Punch, Pursuit, and other Aegislash, and even then King's Shield can easily impede half of them if played right.



HOWEVER, I don't think it should be banned, not at all, it's just too good for OU. Ubers has threats like Blaziken, Mega Gengar, and Ghost/Dark/Fire Extreme Killer Arceus that can all counter it fairly well if played correctly, meanwhile the only true OU counters are Mandibuzz, Earthquake spammers (assuming Aegi isn't running a Weakness Policy set), and Laza Plume, and even then all of those have counters that can completely halt them.