I agree that Aegislash is a much better comparison to MegaGross than Greninja, as both are bulky as opposed to simply blindingly fast and strong, but there are certain characteristics that set the two apart. Bulk is an important one, as Aegislash actually had significantly better bulk due to its typical investment in it as opposed to Metagross' almost prerequisite speed investment. It was also aided by the ability to hold Leftovers, which is recovery Metagross lacks, and King's Shield, which let it possibly neuter physical attackers.
There is no reason in the world why mmeta
can't run hp investment, he certainly has the bulk to get away with it (megazor has very similar stats, barring speed to mmeta and gets away with it just fine, so why not mmeta?). Having a good base speed does not bar him from running defense investment, especially not with defensive stats like his. Lefties and KS are significant though, agreed.
Offensively, Aegislash was also better due to its stats, movepool, and typing. Ghost/Steel is better than Psychic/Steel both for defense and offense, as it gave Aegislash partial STAB on two move perfect neutral coverage. With a ghost type move and Sacred Sword, Aegislash could hit every single Pokemon for at least neutral damage, while Metagross needs all four moveslots. Aegislash could also afford to run either a physical or special set, had access to boosting moves it could afford to run because it covered more in fewer moves, and wasn't hindered by status nearly as much. Burn? Whoops, it was a special set and you gave it a "free" turn. Paralysis? Aegi's slow enough, and it's nowhere near as crippling as it is on MegaGross. On the other hand, MegaGross isn't ever running a special set (ever) so you know what you're dealing with, bar coverage.
Yeah, aegis has some versatility that meta doesn't have, namely the ability to run special sets, setup sets, and sub-toxic. His typing is also much better then mmetas from an offensive standpoint (defensively they're about the same). I think you're overrating his SD set a bit, even at +2 he's not breaking much due to low speed and his poor defenses in blade form. Shadow Sneak helps, but it's still not impressive from anything other then a wallbreaking standpoint.
The idea of Given vs. Taken is good, except when you consider that Metagross will pretty much always be dealing out and taking about the same amount of damage due to lack of extremely useful boosting and healing. Aegislash could be dealing standard damage or it could be running a boosting set that tears though things, and takes somewhere around 120% or 130% damage to kill because of lefties. This doesn't even account for the opportunity cost of using your Mega Slot vs not.
Are the two similar? Yes, but Aegislash is significantly better in multiple ways. There was doubt as to Aegislash's brokenness even now, and you didn't see as much debate about banning despite brokenness. To me, Mega Metagross isn't broken, at least at this point in time.
I will grant that Aegislash is probably more broken then Mega Metagross (though I don't think mmeta's potential is as fully explored as aegis' was, for example there really is no reason why it can't run bulky), and I apologize if I implied otherwise. However, I still think that mmeta still fits my definition of "broken", because he usually
is dealing a fair bit more damage then he is taking.
How to Mega Metagross:
1. Swap him into a resisted hit, taking about 20-30% in the process.
2. If your opponent doesn't have a "good" swap in for his STABs, pick a STAB, dealing anywhere between 50-60% to the swap in.
3. If your opponent does have a good swap in, either A. hit him with a STAB and then follow up with a coverage move, or B. predict the swap and hit with a coverage move. Either choice, in all but a few circumstances, will result in a KO/forceout.
4. If you can't force the opponent out(whether it be through bad prediction or getting countered), just swap out and come back later.
Mmeta can do this 2 or 3 times in a game, due to his constantly mentioned bulk. Each time, he metes out a solid 30-100% worth of damage to the opponent's team. Under most circumstances, mmeta is dealing far more damage then he takes unless his opponent is running one of the handful of mons considered to be his "counter". Even if his opponent is running a counter though, these mons are incredibly passive, allowing mmeta to swap out to one of his dangerous teammates for free, which is always a very powerful move that can outright win games (only a fool would expect mmeta to outright sweep the enemy team, again much like aegislash).
*edit*
Here, have an example:
Situation: phys defensive clefable came out to check your garchomp, forcing him out. You're pretty sure it's not running flamethrower, so you send out mmeta predicting the moonblast:
0 SpA Clefable Moonblast vs. 0 HP / 4 SpD Mega Metagross: 45-54 (14.9 - 17.9%) -- possible 6HKO
Your enemy is running rotom-w and lando-t as potential mmeta checks (nothing else he has is relevent), so you decide to play it safe and go for meteor mash in case of potential flamethrower (which ohkos clefable). Your opponent sends in rotom-w.
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 248 HP / 216+ Def Rotom-W: 37-44 (12.2 - 14.5%) -- possibly the worst move ever
This is pretty bad, to be sure, but lets say for whatever reason you really want him gone (maybe he's stopping a SD talonflame sweep or something), so you Zen headbutt
252 Atk Tough Claws Mega Metagross Zen Headbutt vs. 248 HP / 216+ Def Rotom-W: 133-157 (43.8 - 51.8%) -- guaranteed 3HKO after Leftovers recovery
So by now Rotom has taken about 50% of his health already, so lets assume that he predicts you staying in and goes for the WoW, burning your meta.
However, you're still at an advantage:
252 Atk Tough Claws burned Mega Metagross Zen Headbutt vs. 248 HP / 216+ Def Rotom-W: 66-78 (21.7 - 25.7%) -- possible 5HKO after Leftovers recovery
Remember, Rotom has already taken about 50% damage from MM and Zen Headbutt, so this weakens him further to about 30% of his health, now fairly unable to check much of anything.
Remember that aforementioned SD talonflame?
+2 252 Atk Sharp Beak Talonflame Brave Bird vs. 248 HP / 216+ Def Rotom-W: 121-143 (39.9 - 47.1%) -- guaranteed 3HKO after Leftovers recovery
This is with pretty much perfect play on the opponent's part and virtually 0 prediction on the part of the mmeta user, and the mmeta user is still at an advantage simply by virtue of running mmeta.