This suspect test has been very interesting to go through and despite the fact that I simply don't have the skills to get to reqs (for whatever reason that is, I just can't seem to do it), I've thought long and hard on what my opinion was on Aegislash.
1. Set Dominance
Having played from the beginning of Gen VI, I think its interesting on how the sets have evolved over time. First there was the King's Shield + Swords Dance set, which while it could sweep some teams, proved to be a major flop against most of them and lead several people to believe that Aegislash just wasn't going to be a strong enough threat. Then people started getting the idea of running special moves on Aegislash. For me it was the simple inclusion of Hidden Power [Ice] to get rid of what was, at the time, the most common check to Aegislash: Gliscor. Teams that normally would have been able to deal with the former set no longer could deal with a set that attacked from a different end of the spectrum. I spoke out against the use of Swords Dance, and still do on more defensive sets, and then the meta finally found the move that would make Aegislash a monster to face. Shadow Ball. For someone who used Lando-T for a long time as a Aegislash counter, Shadow Ball proved to pretty much put it to rest. The bulky Spooky Plate set later served to be one of my MVPs, and after that, LO-4 attacks, SubToxic all appeared and we found that while there was a dominant set, variance made Aegislash much more difficult to deal with than before. No longer did you just have to deal with just one set. Now it was three, four, five. All with the ability to just be tweaked to your needs.
2. Lack of Checks and Counters
Aegislash also has, since the inclusion of Shadow Ball into its set, another defining trait: a lack of reliable checks and counters. While many did exist, they weren't always reliable for one reason and one reason only: Shadow Ball is a complete bitch to switch into. The Steel nerf made it so Aegislash had an amazing STAB, coming off an amazing Sp.Atk stat that other Pokemon would only dream of. Yeah, Garchomp checks Aegislash. But can it switch into Shadow Ball? Hell no. Can Landorus? Nope. Can Mega Mawile? Nope. The list goes on and on. Counters were ruled mainly by Mandibuzz, but even then Aegislash adapted and created the terrifying SubToxic set, which easily and totally dealt with Mandibuzz. Quite simply, while each set has its own counters, and checks, its simply uncounterable in the long run. You don't know which set you will face, you don't know if switching Mandibuzz into Aegislash will result in you losing your bird, you don't know if the Life Orb means its about to SD up and wreck face or just blast powerful attacks. What does this mean?
3. The Need to Prepare
Aegislash, quite simply, is a Pokemon you need to prepare for. Back in the day, you could just throw Mandibuzz onto a team and call it a day. Now you can't however. You can't just stick one check or one counter and expect it to work. You need more. If you run Mandibuzz, having something to deal with SubToxic will be incredibly important. If you run Gliscor, having something that can soak up Shadow Balls is very important. If its Heatran, you need a reliable Fighting-resist. Case in point: Preparing for Aegislash requires that you prepare for its wide variety of sets, and that means devoting at least more than one team slot to defeating it.
4. King's Shield
I won't lie, I feared this move would make its way to Ferrothorn and turn it into a monster. Thankfully, Ferrothorn got neither Shield move. ^^; Small tangent aside, this is one of the moves that is fiercely talked about. It's kinda like Protect, except it leaves you with a stinging blow should you make contact with it. To be frank, I think the 50/50 argument is rather bull and its been part of the reason no one always ran Protect. Protect was a liability, and in a lot of ways, it is like King's Shield with its "50/50 scenario." But what I think makes King's Shield bad, in a metagame sense, is that it allows Aegislash to utilize the fullest extent of its stats, while also determining its opponent's moveset. Think back to Gen V and Genesect. What item would you run on Heatran to counter Gene? If you had asked me, I would have said Shed Shell, 100%, as it was the only item that allowed you to counter the bug 100% of the time. Genesect was actually influencing what items you ran, not just your Pokemon. King's Shield does the same thing. People are dropping Close Combat on their Mega Pinsir for Earthquake. Purely for the reason that Earthquake hits Aegislash without having to rely on a contact move. Without King's Shield, Pinsir is bulky and powerful enough that it wouldn't need to run Earthquake. But because of it, its influencing the moveset of your opponent. There are many other examples of this elsewhere, probably too many to list. And while it also make some Pokemon, like Talonflame, who would normally be a good check, very poor checks, the good checks do already run Earthquake or use Fire Blast and that's the issue. The checks are now being decided on what moves you run. Should a Pokemon have the ability to influence not only the Pokemon you use, but the moves you use as well? (Note: Lures are different and I don't think that, for example, Genesect running HP Ground is a real show of Heatran's influence.)
5. Playstyle Influences
This is going to be a short section because it really can't be said that Aegislash actually pushes the meta in one way or another. He can fit onto a large variety of teams, ranging from Hyper Offense to Stall, and so his variety gives some diversity to playstyles. But at its base, I've noticed something. Aegislash's true terror is its ability to make you regret switching. That, to me, is the main playstyle influence it has. Normally, when a dominating Pokemon comes in, your first reaction is to switch out to a more appropriate Pokemon, OR, let that Pokemon die and revenge-kill. As previous suspect tests have shown, often more than not, the Pokemon that are bannable make the first option unviable. Genesect, Torny-T, and Landorus all had U-Turn to immediately switch themselves. Landorus, Mega Lucario, Mega Kangaskhan all made switching unviable by just smashing the switch-in so hard it was crippled. Aegislash falls under the latter group. With just Stealth Rock, Aegislash easily 2HKOes a lot of the metagame. This means that you are pretty much forced to let an answer die to deal with Aegislash. Either it died to give a free switch-in, or it died on the switch to give something else that free switch-in. It speed is an issue, but it easily forces your opponent to drop Pokemon, as switching is just too risky. Normally switching is safe, but Aegislash puts so much pressure you pretty much have to let something die to deal with it.
tl;dr Aegislash has amazing variety in its sets, a move which has not only influenced the choice of checks against it but also the moves they use, no true counters and the ability to beat all of its checks and "soft" counters, and punishes switching heavily enough that switching becomes near non-viable. In my mind, as much as I would not want to see it go myself, as he has always been the glue on my team that has helped it run perfectly, I would be completely okay to see it gone.
(God this post got big where is all of my time)