TOO MANY REVELATIONS!@!!!!
Around the same time in Bleach that Ryuuken shows up to save Ishida and offer him a way of getting his Quincy power back, Isshin shows up in Shinigami gear to save Kon (in Ichigo's body) from an Arrancar. An Arrancar that is Grand Fisher, no less. Remember him? There was a general collective reaction of "...WHAT" from most people - and also from Kon, because there really wasn't anything to suggest that Isshin was more than a normal human. Of course, knowing what we do now, it does make you look make at everything he's said and done in a new light.
So yes, Ichigo's goofy dad is an extremely powerful Shinigami, and this is the first time he has been in his Shinigami form in twenty years. He took down Grand Fisher in one hit, without even releasing his zanpaktou. (I'm not the only one interested in his zanpaktou, right?) We don't know anything regarding his past life as a Shinigami, aside from some hints on his uniform and from a conversation with Urahara, who shows up after Grand Fisher is defeated.
It's quite possible that Isshin has been using one of the untracable gigai that Urahara created. A gigai that drains the wearers reiatsu, similar to the one Rukia had, and that also allows the user to reproduce, resulting in powerful entities like Ichigo? Urahara mentions that it would be "problematic" if Isshin were to blame him if he had become weaker, so he obviously had something to do with it. Isshin was essentially a normal human for the last twenty years - he couldn't save his Masaki from a hollow, something which he says is the one thing he regrets. He also couldn't even see ghosts, if we are trust his word on that. Just what happened to allow him to regain his powers is unknown, unless it was Ichigo's crazy reiatsu spreading like a disease. Again. Isshin's reiatsu is strong enough to get Ichigo and Hiroki's attention, although neither of them recognise it.
Despite the fact that it's only been twenty years since Isshin was in Shinigami form, he has possible been absent from Soul Society longer than that. He was not recognised by Rukia, although it is possible that Rukia - as an unseated officer - would not have known the identity of all the captains. Personally, I think it was unlikely that he was there at the same time as Rukia, but due to the fact that Hiroki does not recognise Isshin's reiatsu, it would appear that Isshin left a lot longer than a century ago. Hiroki was a captain and would therefore have been a Shinigami for a considerable amount of time, and Isshin does not appear in the "Turn Back the Pendulum" flashback chapters. It is possible that that Isshin is linked with the downfall of the Shiba noble house, another mystery that is waiting to be answered. No one in Soul Society recognised the name Kurosaki, and it's likely that he changed his name, or took Masaki's name when living in the human world. Also, when Ryuuken and Isshin met and the former calls him "Kurosaki", the latter is surprised, and comments that this is the first time Ryuuken has addressed him like that. While it's possible Ryuuken only referred to Isshin as "moron" or something similar beforehand, it might just be that he had a different name back in the day. Shiba, perhaps? Ryuuken is a quincy, but he's still human. Obviously he didn't know Isshin over one hundred years ago, so their past is a relatively recent one.
Captain of this ship
Shinigami!Isshin has something interesting wrapped around his arm: something that looks like the tattered remains of a captain's jacket. While we can't see the insignia that would identify the division, it does have the familiar diamond pattern along the bottom - like all the other Gotei 13 captains, and like Urahara (a previous captain). It's interesting to wonder of which division he could have been captain of.
The current captain of the fourth division is Unohana Retsu, who appears to have been the Captain for a long time - at least for a century. She is a calm woman, and remains unruffled by most things that happen. The fourth division are the medics of Soul Society, and our Isshin is a doctor. However, he seems too comfortable in battle to have been captain of this division - the fourth are generally considered useless in a fight. Hanatarou, a seated member of the division, didn't even carry his zanpaktou around - he has no use for it.
Zaraki Kenpachi is the current captain of this division. The eleventh are battle hardened and somewhat bloodthirsty, and they are very loyal to their captain. Zaraki is unusual, both in the manner in which he became captain, and in that he is the only captain not to have achieved bankai. He is incredibly strong, and relies on this strength, and this strength alone in battle. He doesn't even know his zanpaktou's name.
Zaraki became captain after defeating the previous one in battle. Who this captain was is unknown. This battle would have taken place in front of at least 200 members of the division. Isshin is obviously powerful in battle, and could have been captain of this division. However, presumably, the previous captain must be killed? This hasn't been clarified, as far as I know, but if Isshin was defeated, Zaraki is not the type to leave him alive. Unless Isshin had some help with an escape in the form of Urahara. After all, there were 200 people watching.
Or something else?
With the current story arc, we've been hearing things about the royal family, who rule over Soul Society and exist in a separate dimension which can only be opened with the obviously named Royal Key. This royal family is protected by a royal guard (Division Zero), and so far we know next to nothing about them. Only one shinigami - Hikifune, the captain of the Twelfth Division before Urahara - has been named as part of this Division. Even to a Captain, joining the Royal Guard is a promotion. Perhaps Isshin was part of that guard, although one would imagine the royal guard having a different uniform to a normal shinigami - even a captain's uniform, and have one more befitting of their job. When I hear the world royal, I imagine plumes. As it stands, I'd place my bets on Isshin being a former Gotei 13 captain, the uniform he wears is too similar to what we've already seen, even if it is in tatters. Plus, if Isshin was part of the royal guard, or another of Soul Society's forces, it is unlikely he would be in close contact with Urahara, as they seem to work completely seperately from the Gotei 13. Urahara didn't even know of the exsistence of the Royal Guard until he was promoted to Captain.
Regarding the title of this website:
The best panel in the history of the world.
That is all.